I haven't scanned all the posts on this problem so probably am only
restating something already posted.
I will be replacing sections of flooring in my 50's trailers. I would never
have considered OSB or any other composition sheets. They don not stand up
to moisture well at all. I will be using marine grade plywood. I is formed
with water resistant glues. I once had to make a small pool for a tv
commercial I was doing sets for and I used 3/4 inch marine grade plywood. I
used a dry powder marine glue and filler that I bought at a marine supply
store. I mixed a paste to glue the seams of the box and I also made a less
thick slurry which I used to make two coats of filler before I painted it
with marine oil based paint. I sanded between the coats, of course. The
piece never leaked and stayed intact in the back yard for ten years where it
was tossed after the job was done. This experience led me to use the same
system when repairing floor sections. I have done one small entry section
already. I applied a paste to the edges of the marine plywood after I cut
it to fit and then coated the whole piece and sanded it smooth. I painted
the underside and edges with marine paint, leaving the top with just the
sanded filler coat so it would take the floor adhesive. I expect it to last
long after the trailer has been passed on to my heirs. The steel underframe
in an airstream will collect condensation when it is cold and the air has a
lot of moisture in it. Wherever the frame touches plywood you are looking
for dry rot. This is in addition to any rain or plumbing leaks that can
cause problems. So any extra effort to seal out moisture will be well worth
the extra time and expense. Tom Walden 58 Wanderer, 56 Safari & 96 BVan.