I removed and replaced some of my floor in sections. As Bob said, the subfloor is installed underneath the walls. I was able to remove the sections under the walls by cutting fasteners, prying out the old, and sliding new sections underneath the walls. A right angle grinder and reciprocating saw were my friends. I had the walls opened up so attachment through the wall sill was easy.
The original sheathing in my trailer was 5/8" plywood and most of it was OK. Where i had damaged sheets, I replaced with the same. Marine grade sheathing is overkill. It will add considerable weight and expense for small improvement. If you have concerns about future damage to the floor, painting the plywood or sealing with marine varnish should deter that.
One thing I did as an improvement was filling the floor joist cavities with extruded foam. That greatly increased the R-value and comfort level. Since I replaced the original carpet with an underlayment of rubber and laminate flooring, I only had to fill the joints and screw holes lightly with a floor leveler. If an owner chose vinyl as a finished surface, attention to achieving a smooth surface would be necessary.
Good luck with your floor!
