I'll be using three sheets of 4'x8'x5/8" tongue and groove Exposure 1 OSB with PL400 adhesive in the joints. Regarding waterproofing, I will be sealing and edges of the floor with "cut sealant" (paint) and treating the areas inside the wheel wells with sealant and perhaps a can of rock guard. I'm also considering adding mudflaps and a rubber flap 4-6" below the pup's front cross member, to keep the bulk of any road spray from working it's way under the frame and into the bottom edges of the sidewalls while in tow. Judging by the condition of the floor I pulled out, I'd say those are the main areas to focus on. Based on what I've seen just tearing my floor out and according to everything I've read on sealing exposed OSB on RV's, it seems that the experts all agree that it's best to let it "breathe". After having given it a great deal of thought, I see no point in spending the cash to seal the entire bottom side of the floor with anything impermeable. There are just too many areas where water can penetrate the wood. Bolt holes, screws, joints, staples, hatches, doors, vents, trim, windows, the list goes on. Ask any Architect or roofer and they'll all tell you the same thing. No matter how careful you are or how much money you spend on sealants, in our climate, moisture is always going to get in. The smart thing to do is limit the amount and give it somewhere to go. That's my .02 on it, your mileage may vary.