My understanding/experience is that the floor is sandwiched OSB on top, styro-foam in middle, and thin plywood on bottom. Frame rails and outriggers provide all structural support. So deteriorated sections need to be replaced between outriggers, though the foam provides some lateral structure too. Most likely causes of floor rot are leaks along the caulk on top of the exterior belt line, window frames, toilet/sink drains, in that order.
PS--Previous posts on this forum describe a marine treatment chemical that can recover saturated OSB or plywood.
2015 IH45 Foretravel
2003 Alpine 40' MDTS [Sold]
New Braunfels, Texas