Hi, I have difficulty navigating this site but found this thread and thought I'd chime in.
I have a Sunrader shorty that had a rotten spot under the back window almost exactly like the one shown cut out in this thread. My floor panels were coming apart at the seams. a lot of my floor was compromised and ideally should have been cut out and replaced.
I've talked with a couple of guys who have taken the shell off of the truck, added extended braces onto the frame, and cut the entire floor out of the coach and replaced it with new plywood, positioned and bolted back onto the truck frame with the shell set back on top of it and fiberglassed in place all around the edges. In my opinion that's ideal, but in my case I had no place to do that,. Not even a driveway, and had to do my repair in the street where technically I'm not even allowed to change my spark plugs.
What I did was cut out the rot much like was shown here, tied together with nail plates (screwed in place), the places where the old floor panels were separating, and then coated the entire floor with epoxy. After that leveling compound and or One Pass, with epoxy between major applications. After I got everything reasonably level again I cut from 1/2" hardwood plywood a new layer of flooring that went from wall to wall. That had to be done in two pieces, and both pieces were coated on both sides and around all the edges with epoxy before installation. Once I was satisfied with the fit, I drew a grid of 4" squares on both pieces and drilled and countersunk everywhere the lines intersected for deck screws.
I thickened epoxy (with cabosil) and used a 1/4" trawl to spread it onto the old floor and screwed everything down tight using most of a box of deck screws. It came out real nice and will never see water damage again!
My ceiling still needs work, but maybe in another lifetime !
