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For a squirrel to get in there must be a notable hole somewhere....., sure it's not mice?
My first thought would be to check the exterior of the TT.., roof (frig vent, etc), walls (range vent, shore power cord cover, etc.), and complete under carriage of TT, lots of places for critters to get in. Check inside all cabinets around plumbing and electrical openings....., check around the converter/charger wiring as well.
Open the slide and inspect seals for signs of any "chewing", and with slides closed check the corners as well.