I used a 3/4" A/C fir plywood for my floor replacement. I was questioning the supplier about marine grade plywood and explained my application. They suggested the A/C Fir. They said it uses the same marine grade resin in the a/c fir as the the marine grade plywood. The difference being the A/C center cords are made with wood chips as opposed to the marine grade where they use solid wood veneers.
After cutting and fitting my pieces I painted the tops with poly, and the edges and bottoms with primer before installation. After the install it sat in the rain for a few days before I was able to put the shell back on and it looked great. No water damage or swelling what so ever. The water beaded up on the poly and the edges were clean and dry. I'd use the same material again. The unit is near completion and and the floor looks great. I used a commercial sheet vinyl floor covering. I have pictures posted so you can check out my progress and floor and such if you like.
Fair Winds and Following Seas,
But If You're Airstreaming let there be lots of Trees.
1950 Liner 22'
2004 Land Yacht 30 SL
2004 Chevy Avalanche