1978 Trillium 4500 - Rotted Floor. PLZ HELP :)

[HeroSloth]

Hello,

I am new to this forum. I just purchased a Trillium 4500, got it for $4000 CAD, and hoping to budget $2000 of renovations/fixing into it. As it will only be me in the trailer I would like to be as cost-efficient as possible.

The first thing I would like to address is the rotted wooden plywood floor.

I have seen videos and tutorials using epoxy resin with fiber glass cloth, laying the plywood on top then adhering the top layer of fiberglass ontop of the wood and sealing/sanding all the joints.

My questions;

Is the fiberglass cloth completely necessary or can I skip this part?

Is there an alternative/cheaper cloth I could use in its place?

Do I have to put the top layer of fiberglass back on? I was thinking about just laying flooring back on top of the new plywood and sealing all the edges with some sort of adhesive.

I've been reading and it appears I will not be able to use silicon adhesive/sealant to seal the plywood. Are there any alternatives other than epoxy I can use to seal the plywood if I chose to not place the top layer of fiberglass back on?

I understand there are proper ways to complete these jobs, however, the process seems a bit overkill for a trailer as it is not a boat and won't be submerged in water.

Please let me know what you think.

Thanks,

Evan.

[David Tilston]

Evan, Fibreglass mat and resin is not expensive, (https://www.princessauto.com/en/fibr...t/PA0008205007 and https://www.princessauto.com/en/8-sq...t/PA0008205049).

I have put a lot of thought into the job you're undertaking. I did quite a long post on the subject:
https://www.fiberglassrv.com/forums/...tml#post837049

Pultruded grating, (https://www.grainger.ca/en/product/I...FT-/p/WWG4AUC3 and https://www.grainger.ca/en/product/I...FT-/p/WWG4AUH5) would be very expensive, and I doubt you want to go that far.

Were I you, I would take out the fridge, and the bottom fridge vent. Also, I would remove the furnace, if the sliding vent can still slide, if not then just leave it alone. Also remove the converter below the furnace. I would then cut the fibreglass furniture, (kitchen, gaucho and closet) as close to the floor as possible. Also cut the rise up to the rear dinette. A Dremel with a cut off wheel would be perfict for this job. Then grind off the top layer of fibreglass on the floor, and the plywood inside the floor. Use the propper PPE. You would have to get in the kitchen and under the furnace in the closet as well as inside the gaucho. There is balsa wood glassed in on the front curve that transitions from the floor to the front of the trailer. I would leave that alone, as long as it is still in good shape. This would leave the outer layer of fibreglass on the bottom of the trailer as all that is left of the floor.

The rebuild would involve slipping new ¾", plywood in either the door or the fridge vent. Whichever is wider. The plywood should be slid back, along the bare fibreglass, to between the wheel wells. This piece would be very narrow compared to the rest of the pieces. The next piece would be slid forward, under the gaucho, and the last piece would stay where it is slid into place, from the door sill to under the fridge. Personally, I would try to cover the pontoons on either side of the trailer with plywood on both the second and the last piece of plywood. The piece that is between the wheel wells just can't do this.

I have described this as three pieces. It might be more, depending on how wide the plywood pieces end up being.

I would use polyester resin to glue the plywood to the outside layer of fibreglass. It sets fairly quickly, (like in ten minutes) so I would test fit the whole floor first, and then pull out the plywood and glue the pieces in, one at a time. Coat the floor with resin and paint the plywood with resin. then slide the piece you’re working on in. This will push some resin ahead of the plywood. You should wipe that up before it sets.

I would also have something supporting the floor of the trailer from the outside. That way, you can put something heavy on the plywood so that it is pushed together with the fibreglass shell. Actually, I would pull the cab off of the frame and set it on a platform between the pontoons, that would keep the pontoons off the floor.

I would then use pultruded fibreglass angle to attach the furniture to the new plywood, (https://www.grainger.ca/en/product/A...-FT/p/WWG4ATH8)

Then all you have left to do is lay fibreglass on top of the plywood and up to the furniture, then sand and paint.

[HeroSloth]

Hello David,

The resin you posted in the link is what you would recommend, because I would much rather use that over the west system as it is much more cost effective.

If I am reading this correctly, you are saying to chop the bottom of the furniture to accept a 3/4” piece of plywood, then reattach with angles? Or just cut close to the furniture on the ground and sliding it under then support with the angles.

I noticed you did not mention to fibreglass cloth between the plywood and shell. Is the cloth just for the joints?

It also looks like the bolts are completely rusted and they seem to lay on-top of the rotted plywood. Should all this be replaced at the same time? And any idea the hardware I would require?



I appreciate the detailed response. Glad I got the trailer cheap realizing the extent of this work.


Thanks,


Evan

[Frank R]

Take the time to learn how to use epoxy. It is an excellent product for its purposes.

Raka epoxy in Florida has a good product and good prices.

[David Tilston]

Originally Posted by HeroSloth

Hello David,

The resin you posted in the link is what you would recommend, because I would much rather use that over the west system as it is much more cost effective.

The West system is for boats. Trillium made your trailer with polyester resin, like the stuff that I posted a link to. The West system will stick to the existing resin, but then you're stuck with it. The polyester resin will not stick to the epoxy in the West system. People who tell you to use the West system are from a boat background, (which I am) or they have been deceived. DO NOT USE IT.

Originally Posted by HeroSloth

If I am reading this correctly, you are saying to chop the bottom of the furniture to accept a 3/4” piece of plywood, then reattach with angles? Or just cut close to the furniture on the ground and sliding it under then support with the angles.

No. The plywood in the floor is ¾" already. The cut that I'm suggesting is just to keep it clean. Once you grind out the existing rotted plywood, new plywood should have enough room to slide. The fibreglass angle is also just to keep it clean and straight. You could skip this and just fibreglass to the furniture when you glass over the top of the plywood.

Originally Posted by HeroSloth

I noticed you did not mention to fibreglass cloth between the plywood and shell. Is the cloth just for the joints?

The shell is fibreglass. No need for more glass, just glue the plywood to the existing exterior shell, assuming you have cleaned it up.

Originally Posted by HeroSloth

It also looks like the bolts are completely rusted and they seem to lay on-top of the rotted plywood. Should all this be replaced at the same time? And any idea the hardware I would require?

Rusted frame bolts are on the check list of items to address when restoring a Trillium. I used stainless steel bolts with stainless washers which I isolated from the frame with the same sized nylon washers, to help prevent galvanic corrosion. I also put them in upside down compared to the original bolts. That way the threads are inside the trailer. This made the port-a-potty garage a bit more difficult to use due the additional height of the nut.

Other checklist items include:
Belly band re-sealing: Since you have a 4500:
https://www.fiberglassrv.com/forums/...ead-59580.html
But the 1300 thread is also worth looking at:
https://www.fiberglassrv.com/forums/...ead-58763.html

Window wood replacement:
https://www.fiberglassrv.com/forums/...ead-65342.html

Re-hang the door:
Lots of threads on this. Mine is wrong.

Replace the axle:
I put a 3500# Dexter axle on. It was too stiff for the trailer. I would recommend a 2500# to 3000# custom axle, depending on how heavy you want to pack the trailer.

Originally Posted by HeroSloth

I appreciate the detailed response. Glad I got the trailer cheap realizing the extent of this work.

You sure did! Once the work is done, your trailer will easily be worth more than $10,000.

Please post pictures of your trailer.

[theresa p]

Evan----you are VERY lucky to have such detailed information and assistance supplied by David. His experience and knowledge are priceless. Listen to what he says! He knows what he is talking about.



I think you might suggest to friends and family that gift cards to Princess Auto are your preferred Christmas present!



Good luck with your project.



Photos of the process, start to finish, would be fantastic. Post them as you journey along.

[HeroSloth]

Originally Posted by David Tilston

The West system is for boats. Trillium made your trailer with polyester resin, like the stuff that I posted a link to. The West system will stick to the existing resin, but then you're stuck with it. The polyester resin will not stick to the epoxy in the West system. People who tell you to use the West system are from a boat background, (which I am) or they have been deceived. DO NOT USE IT.

Okay I will stray away, the polyester is cheaper regardless, should the 1 gallon tin suffice?


No. The plywood in the floor is ¾" already. The cut that I'm suggesting is just to keep it clean. Once you grind out the existing rotted plywood, new plywood should have enough room to slide. The fibreglass angle is also just to keep it clean and straight. You could skip this and just fibreglass to the furniture when you glass over the top of the plywood.


Sounds good I’ll post pictures so far of how I have cut it all out and stripped the ply wood. My next step is removing the fridge and every other square inch of plywood.


The shell is fibreglass. No need for more glass, just glue the plywood to the existing exterior shell, assuming you have cleaned it up.





Rusted frame bolts are on the check list of items to address when restoring a Trillium. I used stainless steel bolts with stainless washers which I isolated from the frame with the same sized nylon washers, to help prevent galvanic corrosion. I also put them in upside down compared to the original bolts. That way the threads are inside the trailer. This made the port-a-potty garage a bit more difficult to use due the additional height of the nut.


Okay, I will do the same.

Other checklist items include:
Belly band re-sealing: Since you have a 4500:
https://www.fiberglassrv.com/forums/...ead-59580.html
But the 1300 thread is also worth looking at:
https://www.fiberglassrv.com/forums/...ead-58763.html


I have looked into this and am planning on sealing the belly band up in the up coming weeks just have to go buy a dremel!


You sure did! Once the work is done, your trailer will easily be worth more than $10,000.


Yes I’ve seen they are going for a small fortune.

Please post pictures of your trailer.

Once I figure out how to upload I will have some pictures in here



Thanks so much for all the information, I couldn’t ask for better help!

[HeroSloth]

Hello Theresa,

Yes I am extremely lucky and grateful to have David’s wisdom!

[HeroSloth]

[IMG]https://www.fiberglassrv.com/forums/attachment.php?attachmentid=147296&stc=1&d=1663202 370[IMG]

[IMG]https://www.fiberglassrv.com/forums/attachment.php?attachmentid=147297&stc=1&d=1663202 370[IMG]

[IMG]https://www.fiberglassrv.com/forums/attachment.php?attachmentid=147298&stc=1&d=1663202 370[IMG]

The Last 2 photo are how i've cut the fibergalss floor out so far. I believe I may have went a little close to the middle section of the Gaucho while making my cuts. Hopefully I can fit enough resin and fiberglass cloth in there to make it sturdy. If not I will have to use one of those FRP angles.

[HeroSloth]

Here she is and a few photos of the current renovations lol

I believe I may have cut a little too close to the middle section near the Gaucho. Hopefully enough resin and fiber glass will provide enough structure, if not I will have to use the FRP Angle. Also alot more grinding will be required, any tips on how to grind this off quicker, i'm currently using the oscillating saw.

Thanks.

Evan.

[David Tilston]

Originally Posted by HeroSloth

I believe I may have cut a little too close to the middle section near the Gaucho. Hopefully enough resin and fiber glass will provide enough structure, if not I will have to use the FRP Angle. Also alot more grinding will be required, any tips on how to grind this off quicker, i'm currently using the oscillating saw.

Thanks.

Evan.

I would use a 4.5” grinder with a flapper disk:

But be careful, they remove a lot of material quickly.

Since I was suggesting cutting the bottom of the furniture and the rise to the dinette right off, I would say you're fine at the rise to the dinette. I would still cut the angle off at the kitchen, closet and gaucho, unless you can get under what is left, with a grinder. If you have a larger grinder, you will be able to reach further. You will have to remove the safety guard to do that.

My neighbor was laughing at my puny 4.5” battery operated grinder:

He offered to let me use his 7” grinder. I replied, “battery operated tools are enough to get the job done, but not enough to hurt myself too much”. He smiled and pulled up his shirt and showed me the gouge that he cut in his belly when his 7” grinder caught his shirt. Nuff said.

You will also have to remove the fridge to get under it. This is not too difficult. I believe that you have an RM36. The manual can be found here:
https://www.fiberglassrv.com/forums/...do=file&id=180
Once the fridge is out, you will be able to open the bottom exterior fridge vent to help position the plywood pieces.

It appears that Trillium used a similar strategy as I suggested. The plywood between the wheel wells is clearly a separate piece:

The lip that you left at the rise to the dinette will help hold the new piece in place.

As for the praise offered by Theresa, I appreciate your confidence in me, thank you, but please note that while I have put a great deal of thought into this, I have never done this job and I may not always offer helpful advice, but I try.

[David Tilston]

I am fond of looking a pictures of peoples Trillium trailers. I always feel like I learn something. On your pictures I found this:


What is that?

It should be noted that the window guard is frequently missing. These are very difficult to replace. When traveling be sure that it is securely fastened down.

Also, I should add to my list of things that need to be addressed in a Trillium restoration. The propane regulator should be replaced. They are not expensive.

[HeroSloth]

Originally Posted by David Tilston

I would use a 4.5” grinder with a flapper disk:
Attachment 147299
But be careful, they remove a lot of material quickly.

Since I was suggesting cutting the bottom of the furniture and the rise to the dinette right off, I would say you're fine at the rise to the dinette. I would still cut the angle off at the kitchen, closet and gaucho, unless you can get under what is left, with a grinder. If you have a larger grinder, you will be able to reach further. You will have to remove the safety guard to do that.

Hello Dave, do you mind illustrating in a photo with the red marker what you are referring to as the "rise to the dinette" and "angles at kitchen, closet, and gaucho"? I was also wondering how a piece of wood will be able to bend to slide under both the wheel wells. Do you know if theres a diagram showing the original sections of plywood layout/area? Thanks agian. Evan.

Also, I will have to make sure to keep that window guard treasured lol.

That plywood seam looks to be where I saw cut across to remove the floor section easier as it was not coming up in 1 piece.

[David Tilston]

Originally Posted by HeroSloth

Hello Dave, do you mind illustrating in a photo with the red marker what you are referring to as the "rise to the dinette" and "angles at kitchen, closet, and gaucho"? I was also wondering how a piece of wood will be able to bend to slide under both the wheel wells. Do you know if theres a diagram showing the original sections of plywood layout/area? Thanks agian. Evan.

Also, I will have to make sure to keep that window guard treasured lol.

That plywood seam looks to be where I saw cut across to remove the floor section easier as it was not coming up in 1 piece.

Below are mark ups of your pictures pointing out what I was calling angles or probably better called tabs. The only one I would leave is on the rise to the dinette, (also labeled):


I'm not aware of any diagrams of the original plywood arrangement. I'm not sure that it was done in any consistent way. It likely differed with each installer.

Thanks for the correction about the seam. It should not come up in one piece. If it does, I would consider that a failure of the glue.

I don't understand your question about bending the plywood under the wheel wells. The plywood should extend from one wheel well to the other, under the kitchen cupboard and under the bottom of the closet. This is why I would cut the tabs off of both and grind the remaining wood till you can slide a piece of plywood that is wide enough to reach from the vertical inner wall of each wheel well to the other. Basically, the distance of the arrow labeled, "Wheel Wells".

The piece that would be butted up to that piece would start under the forward wall of the closet and would extend to the door sill on one side. The other side would go, under the kitchen, to the curb side wall of the trailer. Trillium stopped before connecting to the far wall. There may be a good reason for this, but I can't figure it out. I would think that attaching the plywood to the far wall, over the pontoon, would help keep the wall from sagging. I would also glass the top of the plywood, including the part that goes under the closet and kitchen. If you do glass it to the walls of the trailer, I would drill drain holes in the plywood, lined with some fibreglass.

If you have not yet done so, you should also drill ¼" holes in the pontoons, directly behind and in front of the wheel wells, at the lowest points in the pontoons. This will allow any water that collects in the trailer to drain out the bottom. The rot that you are fixing is likely the result of water collecting in the pontoons and soaking the plywood. When I drilled these holes in the six Trilliums that I have owned, a surprising amount of water came out. This is another thing that Trillium should have done when they manufactured the trailer. There is a posting somewhere on this site where someone drilled the holes and put in an insert that had a bug screen. Sounds like a good idea to me.

[HeroSloth]

Hello David,

Thanks for the clarification. Just brainstorming here, I guess my only question/concern is, once I cut all of the angles off, will the resin and mesh be strong enough to recreate the structure at these angle joints? This is the main reason I left the tabs, it might be a pain in the ass to get the wood out from under, however, I do plan on laying the fiber glass back ontop and re-sealing everything it. The fiber glass work to recreate this and sand it all out will also appear much higher than the flooring as it will be feathered out onto the wall I believe. However what do I know, I’ve never done fiber glass work.


If I cut the tabs I might just have to lay the plywood down and rather than putting fiber glass ontop, lay the waterproof flooring directly on the plywood.


Let me know what you think.


Thanks Again,


Evan.

[David Tilston]

Originally Posted by HeroSloth

Hello David,

Thanks for the clarification. Just brainstorming here, I guess my only question/concern is, once I cut all of the angles off, will the resin and mesh be strong enough to recreate the structure at these angle joints? This is the main reason I left the tabs, it might be a pain in the ass to get the wood out from under, however, I do plan on laying the fiber glass back ontop and re-sealing everything it. The fiber glass work to recreate this and sand it all out will also appear much higher than the flooring as it will be feathered out onto the wall I believe. However what do I know, I’ve never done fiber glass work.


If I cut the tabs I might just have to lay the plywood down and rather than putting fiber glass ontop, lay the waterproof flooring directly on the plywood.


Let me know what you think.


Thanks Again,


Evan.

I like fibreglass. Gluing the plywood to the outside fibreglass shell and then fibreglassing the top of the plywood makes a sandwich that is much stronger than the individual components. Kind of important for the floor. Many fibreglass boats use a foam sandwich construction, (yup that’s what it’s called). Plywood is not foam, but I think the same principle applies. Many people install a vinyl click style floor over the fibreglass. It is not affected by water, and it is easy to install. Since it is not glued, it can expand and contract without warping:
https://www.homedepot.ca/en/home/cat...ank/f/pra-2kg7

Also, if you feather the edge of the fibreglass over about 1" to 2", from full thickness down to paper thin, this gives you enough surface area for a strong connection. See my belly band threads. That way there is no additional thickness. If you cut the tabs off, I would just fibreglass the back, (inside) of the furniture to the plywood and fibreglass covering the plywood. Invisible. That is why I would use the pultruded fibreglass angel. It would ensure a straight line and if installed on the back, (inside) of the furniture face, it would be invisible.

[HeroSloth]

Hello David,

So I am leaning toward making these cuts, I wish I had done it from the start now I have twice as much fiber glass work to do lmao.

I have attached two images; the red lines are where ill be making my cuts, the blue is everything I don't plan on touching.

Can I get your professional opinion on this haha?

There also a progress photo of under the fridge, still needs to be grinded.








Thanks as always!

Evan.

[HeroSloth]

Originally Posted by David Tilston

I like fibreglass. Gluing the plywood to the outside fibreglass shell and then fibreglassing the top of the plywood makes a sandwich that is much stronger than the individual components. Kind of important for the floor. Many fibreglass boats use a foam sandwich construction, (yup that’s what it’s called). Plywood is not foam, but I think the same principle applies. Many people install a vinyl click style floor over the fibreglass. It is not affected by water, and it is easy to install. Since it is not glued, it can expand and contract without warping:
https://www.homedepot.ca/en/home/cat...ank/f/pra-2kg7

Also, if you feather the edge of the fibreglass over about 1" to 2", from full thickness down to paper thin, this gives you enough surface area for a strong connection. See my belly band threads. That way there is no additional thickness. If you cut the tabs off, I would just fibreglass the back, (inside) of the furniture to the plywood and fibreglass covering the plywood. Invisible. That is why I would use the pultruded fibreglass angel. It would ensure a straight line and if installed on the back, (inside) of the furniture face, it would be invisible.

David,

Having a look at it today, you were correct that straight line was the OEM plywood seam. I have cut all the angles off now, a lot of them kind of cracked to 100 pieces so I’ll have to glass cloth those areas. Been having a fun day chiseling all the soggy wood from underneath lmao.

The only angle I left was the one at the front door. I’ve got rid of the rise to the dinette angle.


Thanks again,


Evan

[David Tilston]

Sorry I have not responded sooner. I was on a trip to Vancouver Island to buy my last Trillium:
https://www.fiberglassrv.com/forums/...tml#post848534

What did you use for the cuts? Why did you remove the rise to the dinette? Was that one of the parts that cracked to 100 pieces? What I plan to use for the cut is a Dremel style tool with a cut-off wheel. Slow, but gentle.

A chisel sounds like a hard way to remove all that plywood, but if that is what you feel comfortable with, you do you.

So, to the job at hand:
Why are you not removing the tab between the doors on the gaucho? The plywood in the gaucho is helping hold up the front curb side of the trailer. The door represents a major structural flaw. If the plywood in the gaucho is pristine, I can see why you would leave it. I have seen several Trilliums that sag on the front curb side. This can be observed by standing on the tongue of the trailer and looking down the flat section of roof over the door. If the closet forms a high point on that section, then sag is a problem. Another indication of front curb side sag, which will be difficult at this point, (since there is no floor) but for reference, put a straight edge on the floor from the front of the fridge to the door. If you see the floor curving down towards the door, that is sag. Having a solid piece of plywood that spans across the gaucho is essential in my opinion. With that tab in place, sliding a full width piece of plywood into that area will be very difficult.

Just out of curiosity, what is wider, the fridge cubby, or the door? Also, how well does the rear side wall of the cubby line up with the closet wall that is beside the door? The reason I ask is that it affects the strategy of how to lay the plywood, after the piece between the wheel wells is down. I am now leaning towards a piece that would go in the door, standing on the long edge. Insert that piece into in the fridge cubby and then lay it down. If the tab at the door is left in place, then you will have to slide the plywood under that tab. This means that at the curb side of the trailer, the plywood will not connect with the wall, over the gaucho. Since Trillium left the gaucho exposed in the fridge compartment, (and elsewhere) this may not be a problem, but in my opinion, attaching the plywood to the exterior walls of the trailer would be stronger. However, I would then drill regularly spaced 1” holes, in the plywood, over the pontoons, for drainage. While doing this job, I would also replace the wood in the gaucho. I’m not sure if you plan to replace that plywood, but if you are, the first piece would slide forward, under the gaucho, through the space that is currently occupied by the previously mentioned tab between the gaucho doors. Also, you will need to remove the four frame bolts that are in the gaucho, (they need replacing anyway). Counter to my previous advice, I would not glue this piece to the exterior shell yet. If this piece is not wide enough to go far enough under the gaucho, then it would make sense to use two pieces of the same width to make up the required width. The next, (last) piece would go in the same way, but before it is laid down, coat the exterior shell and the underside of the plywood with polyester resin. lay it down and slide it back slightly till it connects with the piece of plywood between the wheel wells. Now lift the piece,(s) in the gaucho and with some difficulty coat the bottom of the plywood and the exterior shell with resin. Ideally the cab of the trailer would have been removed from the frame and it would be sitting on a perfectly flat platform, between the pontoons that was high enough to keep the pontoons off the ground. That way you could fill the trailer with 5-gallon pails filled with water. This is to provide the down force for good contact between the glued parts. It could be done with the cab on the frame, but the clamping force would be uneven. It would be less where there’s no frame sections. When the resin sets I would then fibreglass over the plywood. Including inside the furniture. Also, fiberglassing the furniture to the floor at the same time.

Once again, I have never done this job, so take what I tell you with a grain of salt.

[HeroSloth]

Originally Posted by David Tilston

Sorry I have not responded sooner. I was on a trip to Vancouver Island to buy my last Trillium:
https://www.fiberglassrv.com/forums/...tml#post848534

What did you use for the cuts? Why did you remove the rise to the dinette? Was that one of the parts that cracked to 100 pieces? What I plan to use for the cut is a Dremel style tool with a cut-off wheel. Slow, but gentle.

A chisel sounds like a hard way to remove all that plywood, but if that is what you feel comfortable with, you do you.

So, to the job at hand:
Why are you not removing the tab between the doors on the gaucho? The plywood in the gaucho is helping hold up the front curb side of the trailer. The door represents a major structural flaw. If the plywood in the gaucho is pristine, I can see why you would leave it. I have seen several Trilliums that sag on the front curb side. This can be observed by standing on the tongue of the trailer and looking down the flat section of roof over the door. If the closet forms a high point on that section, then sag is a problem. Another indication of front curb side sag, which will be difficult at this point, (since there is no floor) but for reference, put a straight edge on the floor from the front of the fridge to the door. If you see the floor curving down towards the door, that is sag. Having a solid piece of plywood that spans across the gaucho is essential in my opinion. With that tab in place, sliding a full width piece of plywood into that area will be very difficult.

Just out of curiosity, what is wider, the fridge cubby, or the door? Also, how well does the rear side wall of the cubby line up with the closet wall that is beside the door? The reason I ask is that it affects the strategy of how to lay the plywood, after the piece between the wheel wells is down. I am now leaning towards a piece that would go in the door, standing on the long edge. Insert that piece into in the fridge cubby and then lay it down. If the tab at the door is left in place, then you will have to slide the plywood under that tab. This means that at the curb side of the trailer, the plywood will not connect with the wall, over the gaucho. Since Trillium left the gaucho exposed in the fridge compartment, (and elsewhere) this may not be a problem, but in my opinion, attaching the plywood to the exterior walls of the trailer would be stronger. However, I would then drill regularly spaced 1” holes, in the plywood, over the pontoons, for drainage. While doing this job, I would also replace the wood in the gaucho. I’m not sure if you plan to replace that plywood, but if you are, the first piece would slide forward, under the gaucho, through the space that is currently occupied by the previously mentioned tab between the gaucho doors. Also, you will need to remove the four frame bolts that are in the gaucho, (they need replacing anyway). Counter to my previous advice, I would not glue this piece to the exterior shell yet. If this piece is not wide enough to go far enough under the gaucho, then it would make sense to use two pieces of the same width to make up the required width. The next, (last) piece would go in the same way, but before it is laid down, coat the exterior shell and the underside of the plywood with polyester resin. lay it down and slide it back slightly till it connects with the piece of plywood between the wheel wells. Now lift the piece,(s) in the gaucho and with some difficulty coat the bottom of the plywood and the exterior shell with resin. Ideally the cab of the trailer would have been removed from the frame and it would be sitting on a perfectly flat platform, between the pontoons that was high enough to keep the pontoons off the ground. That way you could fill the trailer with 5-gallon pails filled with water. This is to provide the down force for good contact between the glued parts. It could be done with the cab on the frame, but the clamping force would be uneven. It would be less where there’s no frame sections. When the resin sets I would then fibreglass over the plywood. Including inside the furniture. Also, fiberglassing the furniture to the floor at the same time.

Once again, I have never done this job, so take what I tell you with a grain of salt.

I will probably have all the wood in this weekend. I plan on placing weight on the plywood from above and considering using a few piece of plywood underneath held up by car jacks to ensure the bottom side of the trailer is getting equal adhesion to the resin.

I used an oscillating saw for the cuts, worked perfectly. Dremel might be a bit more efficient, I do not own though. The chisel was used to remove the wood which slide under all the cabinets, rise, etc by a few inches. Pain in the ass. The remaining wood will be sanded off with one of those flappy disks you showed me.

I will send you pictures of how I end up tacking the plywood, I do believe I will probably do it in 4 sections. 1 between the wheel wells sliding into the rise. 2 under the gaucho, the second piece will slide under the fridge section by half a foot to a foot. Lastly, slide a piece under the remaining part of the fridge and cabinet/furnace space which will join the first piece between the wheel wells.

Heres an illustration, obviously not to scale but should work. I might have missed a few small obstacles.
The black illustrates the Plywood outline.
The red illustrates obstacles to fit under.