How to treat a Subfloor?

GrandpaKip wrote:
Marine grade plywood is overkill in my opinion. Exterior AC plywood would work just as well, is less expensive and easier to find. Not necessary to seal it, either.
I would use foam board for insulation if you think it’s necessary. Since we don’t camp in the cold, I would skip it.
Coroplast, a corrugated plastic sheet, can be used to enclose the underbelly.

Some more lack of understanding of plywood grades, types, and properties. Here.
Why spend the big $ for AC for sub flooring. What's the advantage to an A grade sanded surface that's getting covered with flooring? Especially in an old camper.

Agree marine grade is 100% unnecessary unless you're pre planning for leaks that will rot your floor out 15 years from now. But if you insist on pressure treated flooring in a "dry" application, there are cheaper options that what folks are referring to (AB marine grade).
Back to the Tn G thing. Unless you're a good carpenter, I see installing t n G being a challenge in a RV rehab, unless the whole thing is gutted out square box and even then you'll probably lose the tongue on the last piece to fit it.

Don't overthink it. 3/4 CDX plywood is more than sufficient. Or Advantech if you can find it reasonably priced.
One consideration is joist spacing and what was in there and how much the floor flexed or will flex. You may want to orient the sheeting a different direction to prevent any seams in high traffic areas or consider better quality ply (7 or 9 ply) for high traffic area.

So coming full circle, if you're getting AC for the stiffness of it, may be worth it. Only need a few sheets anyway, correct?
I don't have plywood plies by grade memorized, but for the same thickness board, more plies = stiffer. And less flex.