Hell, I'm using $10 / sheet Luaun plywood, with insulation and pine 1x3's sandwiched inbetween.
Anything will rot eventually. It's inevitable. I'll coat my exterior with epoxy, then truck bedliner or automotive enamel.
My old cargo trailer is at least 30 years old, and the untreated floor that was never sealed by anything is still rock-solid.
All factory-built campers don't use marine-grade ply. I don't see why you'd need it.
