1999 Allegro Bus sub-floor replacement

[Cpvest2]

Inherited my coach from my ailing father for use during racing season. Long story short, live 11 hours away and he put coach in storage for past 4 years when his health got worse. Neither my mom nor dad checked inside the thing for those 4 years. I finally got time to get back and check on it and discovered a mess.

Skylight to roof seal was nonexistent and allowing a stream of water into shower/roof/wall. (There appears to have been a repair done and an incorrect skylight size was used because the skylight doesn’t fully cover the hole in roof)

Long story short, I am cleaning up the mess. I have the top layer of sub floor removed but there is moisture/rotten wood under the foam as I can flex the foam considerably. how do I go about replacing the osb under the foam and metal framing? Do I need to “drop” basement to do it properly? I’m assuming the osb I need to replace is actually the ceiling of the basement compartment.

[Cpvest2]

I really would like to save this thing as mechanically the coach is in great shape, just needs some TLC to roof, floor and a good remodel.