How to treat a Subfloor?

GrandpaKip wrote:
Most contemporary trailers use the plastic corrugated sheets as the bottom layer that is closest to the road. No sealing, no rot and lower cost. Most people call it an underbelly cover. I have bought some at Home Depot in the past.
I suggest you go take a peek under a trailer at an RV dealer.

^ This is what I was describing, but appears the OP is intending to make a plywood sandwich with foam insulation in the middle on top of the floor framing.
Is this how the trailer was originally built? If not, have you accounted for the additional floor thickness or height?

Bottom line, the water resistance of the bottom plywood is being scrutinized far too closely, IMO. It is not submerged, it is not constantly exposed to the weather from above. It only gets wet while driving in the rain and then is sheltered and dries out.
For example, I have a "temporary" plywood ramp I built into the back of the shop about 9 years ago. wood frame, don't remember if I used treated lumber or not, with a piece of MDO plywood on top. It has been 100% exposed to the wettest weather (Seattle) from above and has yet to rot out.
Pretty much all cargo trailers have exposed plywood on the bottom, some are pressure treated, some traditional plywood or other material like Advantech (exterior grade OSB basically). Have a 15 year old trailer that runs only in the winter (wet/salt/slush) and sits outside. Floor is fine, like new still.
Any exterior glue plywood with a coat of paint, stain, varnish, waterproofing sealer will likely out last the trailer again as it is only occasionally exposed to road spray and sheltered the vast majority of the time.
Barring re-constructing the floor the way RVs are typically built. 1 layer of flooring above the joists, insulation in between the joists and a thin "skin" under the joists, if your doing the sandwich floor...
I'd think 1/2" ply bottom, 1" 30-40psi foam board, 1/2" plywood on top.