One it can be repaired. You are essentially:
- Cutting a piece of plywood, coating one side with FG resin using a paint brush
- Setting it on the frame, screwing it down
- Then using fiberglass and resin like duct tape to tape the floor to the wall. Top and bottom. Attaching like this with fiberglass is called "tabbing".
- Painting the floor.
Removal is just the reverse. Cut the fiberglass and resin tape that joins wall and floor (those oscillating saws work well) use a grinder to grind the heads off of the existing screws that hold the floor to the frame. Pull up the piece of rotted floor.
Propping the shell up during the operation by using 2x4 or 2x6 boards wedged under the belly band. Can also brace against roof from floor inside.
floor is in sections so you can do one section at a time. Me I would start with one furthest from the door.
As an alternative you can cut through a patch piece of wood and the rotted floor at same time and the patch will drop right in. An exact match for the hole.
Remove the old floor piece you just cut through just as you would for replacing whole floor as outlined above. Then cover bottom of hole with aluminum foil and screw a board across the hole from the underside for the patch to rest on. From there it is just use FG and resin to attach the patch.
Look out for rot at the bottom corners of the door. The floor, frame, wall and door frame joint can become loose from rot there. Allows the door frame to shift and door will look crooked in opening. Been there done that.
If you totally have no skills with tools such as a power saber or scroll saw then you will have to find a friend, family member or as a last resort a paid handyman to do the work. Watch and learn. You see one done you can probably muddle through the next one, do a decent job on the third piece and be a freaking expert by the next piece.
That and this community is very supportive. You start a thread on your repairs in progress and you will find a whole lot of members will help you over any speed bumps.
I have plenty of experience with tools and such but replacing floor and working with FG were new to me. Between the advice from the members and the stuff I found on YouTube, or they found for me I managed ok.
Below note the small lip of fiberglass tabbing from the underside of the floor left in place when I cut the floor free. It is visible in the first and second picture here. That gives a lip to set the new floor on. Once I'm ready to drop in the new floor I coat the lip with fiberglass resin and slip it into place on top of that old lip. I think one could also use Epoxy or construction adhesive.
Also note in top frame the piece of wood under the floor that attaches the bottom of the wall and anchors the base of the door frame. Mine was rotted. Door frame could move at that bottom corner.
Last picture frame shows how I dropped in the floor. There is Liquid Nails construction adhesive going on top of the frame so this prop allows me to drop the wood right on it. I then had to cut the center piece out. The construction adhesive prevents that cut out from leaking. And avoids squeaks too.
Eventually to this. Note the liner rolled up, the wood strip that supports the back of the couch was also rotted and had to be replaced by tabbing in with fiberglass.
This is doable with basic skills and ability to work slowly and carefully. Willingness to ask questions helps. A lot.