Looking at your photos I can tell that the OSB flooring is beyond saving and is falling apart. So you can bandaid it or replace the floor, the manufacturer used the cheapest OSB board available so it was a problem waiting to happen. If you replace it and use OSB go to a lumber yard not a big box store and buy some high quality OSB sheeting or plywood, you may have to laminate 2 pieces together to get the right thickness. Cover the underside with fabric that higher end units use to keep the wood dry and use aluminum flashing to seal the edges. You may want to go to a dealer and look at how other manufactures seal theirs.
Denny
