waterproofing underside of cabover

Also, is there any reason I shouldn't use plywood for the entire floor. Originally, it was built like a hollow door, stuffed with Styrofoam. The plywood will weigh more, but wii be stronger, right?

The plywood might be somewhat stronger, but laminated plywood/foam is remarkably strong and stiff. The foam construction would have better insulating properties (or at least less bad ones), and should help prevent some condensation in that area. The plywood will also weigh a good bit more, as you mention, which can cause its own problems. But on the whole, I think either one should be workable though I would prefer the composite construction.

Maybe you could get some countertop laminate sheets from a local home center to cover the bottom with? My gut feeling is that perfect hermetical sealing is not entirely necessary here, as there's comparatively little chance for water to work its way backwards/upwards, and it's certainly not going to puddle on the underside. The main exception may be if driving at speed in a torrential downpour, but who really cares to do that regularly? You might even do OK with a good quality exterior paint or stain and good caulking of the seams.

I would suggest using marine plywood, or at the very least exterior grade plywood. It's much more resistant to falling apart when it gets wet. Interior grade plywood does not do well when it gets wet.

I used exterior 3/4 to replace my floor and also sealed it with CPES (clear penetrating epoxy sealant). Mine was originally wood, though.

The Styrofoam would insulate much better and be lighter.

Jose

I used 3/4 exterior ply for the floor and applied 3 coats of deck paint to all surfaces. For the skin I used 4x8 sheets of fiberglass from a builders surplus store. This is the same material used for the wall of a shower stall. Cost $12 each. The advantage in my opinion of the fiberglass sheet over laminate is the flexibility. You're not going to get the laminate to wrap around the nose of the overhang if that's what you need to do. And, it's waterproof inside and out. When working with the fiberglass I used 1" 3M VHB tape to adhere it to the frame then instead of re-installing the aluminum edges I used 3 layers of 4" fiberglass tape and MAS epoxy (used in wooden boat building).

Okay, so I would like to go with 3/4 ply but my dimensions are 98" x 54" (interior 96" x 54"). Did you guys have the same dimensions? If so, did you piece together the plywood or were you able to find sheets larger than 4 x 8? If you did piece them together, what was your method.