I have seen racoons chew through the plywood floor of an rv but your problem sounds more like a squirrel or chimp monk etc. I would guess that they may have come in around the drain pipes and in up around the tanks and basement area?
My FW had openings around the drain pipe and into the tank area with gaps around the vent pipes that a squirrel could have easily gotten through. Look under the rig to see if there is an area big enough for them to fit through. I spray foamed everything. I noticed spray foam at the hardware store that is used for sealing out rodents/pests. I dont know what it contains but it said no pesticides.
It's surprising what some animals can chew through. We had a small dog that was afraid of storms and it chewed through an aluminum dryer vent pipe.