I have had both a '73 1300 and a '78 4500 and both had similar issues from similar problems.
In the Trill the floor is a sandwich of Fiberglass belly outside,3/4" plywood as the "Meat" and then a fiberglass tub inside that serves as the floor inside where you put your feat. The inside tub also has the benches as part of it all molded as one huge part sort of.
I word it like that for a reason,the plywood meat in the middle gives the fiberglass a base on both sides making the floor strong for sure but then the plywood extends beyond the inner tub on both ends and is visible in the underseat storage areas on both ends.
On the tongue end it is the exposed floor in that compartment and is almost the same in the back but doesn't go quite to the side walls back there.
If you notice the rear side below floor level looks like a 'U" really and it is fairly obvious. The same is true in the front but it is a little less obvious.
I have to think these are really there as a channel for water to run under and away from the inside floor area in case water does get in and start to pool up?
I tried and never found any weep holes where the water could leave these channels in either of my Trill so I ended up drilling my own and in both cases water poured out for a long time.
I don't know how long it had been in there but you don't really want it at all and who knows what starts living and growing in there,it seems like a perfect breeding ground for nasty things. On top of that as is mentioned above Dry Rot is caused by Moisture(ironic?) and this is how it starts.
On my 1300 the plywood did not look too bad but on the 4500 it did. The floor was solid though and tough to get too without gutting the trailer which was out of the question for me then so I sought to find the cause and fix the problem there.
Luckily maybe it was the same in both rigs.
I ripped the windows
out,replace the framing which holds them in and re-installed them using Butyl rubber and stainless steel fasteners.......Problem Solved!
I also had to repair the Belly Band on the 4500 which is a common issue I needn't go into again here but the bottom line was that once I removed the problem all was dry again.
I then drilled a million 1/8" holes in the exposed plywood in the storage compartments and rolled on a really runny epoxy/resin to cover the somewhat dry rotted wood and once dried I screwed on an indoor/outdoor carpet to cover it up.
I don't think this did much really but it did seal the dry rot in? and provide a cleaner surface in the storage areas.
My original point was that because it is sealed underneath,this may actually further the problem unless you are careful to look for it going in.
Now the Casita
does not seem to be designed with these channels for drainage? I am not sure Trillium
didn't have a great idea and design it in and overall I found the design and build of the Trill to be amazing compared to other egg's I have seen.
I don't know if I have any pics as it has been a while since I did all of this but if you have a Trill and go look at it you will quickly see what I am talking about.
Hope this helps.