Actually I am in the middle of my build.
I have decided to fiberglass tape the joints and corners and roll epoxy resin over all the plywood.
When that cures, I'll shoot it with Kilz.
When that cures, I'll shoot it with a porch & floor paint.
I chose Porch & Floor paint because I figured if it's good enough for foot traffic on outdoor porches and decks, it should do pretty good protecting my trailer.
Time will tell the tale.
I am ot trying to be fancy, just functional.
jj2k