When I redid my Scamp
I fiberglassed the floor sections at one time after trial fitting all over. The top, bottom and sides and the fiberglassed the floor to the sidewall.
I also sealed all of the many no longer needed openings etc and now I dare the bugs to get in (not really, but IF I can convince Connie (She Who Must Be Obeyed) then all is good).
The main cause for the rot in my opinion (at least with my Scamp) is leaks
inside the the trailer and the water standing. I did not see anything I thought started on the bottom. However fiberglassing the whole thing should take care of it at a higher cost, however. I also sealed (glued) the floor down on the frame with construction glue to keep water from being trapped between the structure and the wood and to add stiffness.
I have used one part clear epoxy garage floor "paint" to good effect to seal the plywood I used elsewhere in my project. Two coats builds a good finish. If you thin it a little with some acetone it will soak into the wood better for the first coat.