Soft bathroom floor replacement

I would not use pressure treated plywood on the top layer. It is hazardous and vapors will permeate the vinyl into the living area. Cut everything as straight and square as possible. Use structural adhesive to attach the plywood to the supports, along with screws. I would use ridged foam board instead of all spray foam. Fit everything as close as possible before starting assembly. Use a toothed trowel to spread a foam compatible construction adhesive for the foam. I would use marine grade plywood for the top subfloor. Once you have it fitted, I would give it at least 2 coats of shellac to water seal it, so you do not have the same problem at a later date. I may catch flack for saying shellac, but it is the only finish that is water proof and will hold up.

Take off one of your floor vents and you can usually see how your floor is built. For that year HR I am guessing that your floor is built using sandwich construction with foam in between layers of OSB. The sandwich, may be 2-3" thick and is set on a steel framework. If that is the case, you may have to remove all of the OSB, foam, and the bottom layer against the frame before you can start building it back up. How bad the bottom layer against the steel is will determine the best way to complete the repair. You can use spray foam to replace the damaged foam and once that is at the proper level, then you can glue a new top layer of plywood.

I replaced the floor in a 5'er that I had. There were beams underneath the OSB in it. I would think there would also be something to attach the flooring to there. It was attached originally so I would think it could be reattached. You could probably call the manufacturer and ask them what's down there.