How to treat a Subfloor?

Marine grade plywood is really good stuff, but it's not waterproof, nor is it treated. It is high grade plywood, five to seven ply, sanded on both sides, with no voids allowed. The glue is waterproof.

It is also horribly expensive.

Most CDX is not suitable for vinyl sheet flooring (telegraphed defects) and has a twist that can difficult to deal with in close quarters.
BC plywood can also be used, though not as flat or smooth as AC.

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.

This is what I would use for insulation:

https://www.homedepot.com/p/SilveRboard-0-625-in-x-48-in-x-24-in-72-sq-ft-R-3-Graphite-Radiant-Barrier-Wall-Insulation-Kit-9-Sheets-Kit-SBGXS062502PKIT/306858613?mtc=Shopping-VF-F_Vendor-G-D22-22_4_INSULATION-Generic-NA-Feed-PLA-NA-NA-INSULATION_GeneralInsulation&cm_mmc=Shopping-VF-F_Vendor-G-D22-22_4_INSULATION-Generic-NA-Feed-PLA-NA-NA-INSULATION_GeneralInsulation-71700000033102370-58700003867202295-92700048704098264&gclid=Cj0KCQiAl5zwBRCTARIsAIrukdO3cSEGCsOT9H_fPYqsXirv1g47mupS8jC7xL0hM3rTnMbD4PxoNDwaAsWCEALw_wcB&gclsrc=aw.ds

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.

It may be worth while to go on one of the factory tours--to ask questions about how RV's are built.

Thank you so so much! It seems like such a simple answer but I've had an incredibly hard time finding information and there's a lot of misinformation out there too. Really appreciate the responses. We'll be going out to buy the marine grade plywood for sure

No, you got it right.
The flooring you put on top of the framing (under the vinyl floor) is the sub floor. As it should not get wet from the elements at all, does not need to be marine grade or treated or anything of the sort. I wouldn’t use OSB as future leaks from plumbing could destroy OSB rather quickly.
By tongue and groove, presume you mean t&g plywood? Tng wood planks might work as well, but more considerations.
The underbelly or skin under the insulation and framing should be marine grade or painted (including the ends) as this sees all the road spray traveling in the rain.
I wouldn’t use wood down there at all. Except maybe cheaper to use 1/4” plywood and paint. Plastic sheeting, filon, bathroom tub surround sheeting, would be more preferable. Not only for moisture intrusion resistance, but you don’t need any “strength” down there. Just something to cover up the insulation. Lighter weight the better.
Good luck with the project!

Okay I think I'm getting my terminology confused. Is the subfloor what the vinyl (or finished flooring) attaches to, or is it the base which is exposed on the outside of the Camper?

I would strongly suggest marine grade materials for the repair. Return what you purchased to the store.

I would lean towards tearing down right to the skin and installing marine plywood as the sub floor.

On top, I'd use one of the non tongue in groove plastic (thick version) floors.

Consider adding a heated floor.

So, make a sandwich:

finish floor (thick plastic NOT tongue in groove)
floor heating
marine plywood
insulation (silver foil)
seal at edges with great stuff or similar foam.

This is a job that will only be done once. Do not skimp on materials cost.