"What causes soft floors"
In my case of my past thor, it was wicking up from underneath most likely caused from condensation. Tailer had no delamation, no roof leaks, nothing anywhere I could find a leak source. Floor construction: starting from down going up was very thin sheet metal,lauan,styrofoam, lauan then topped with linoleum all glued.
I ended up dremming out patches of the sheet metal, replaced with bug screen. This allowed the wood to dry out underneath and on the top side I cut out the linoleum, allowed the wood to dry out and topped with new plywood and those stick on linoleum tiles. This was not a cure all but certainly a heck of a good remission repair. I ended up getting 11 years out of a poorly constructed trailer.
Mike