No, they have to be getting in somewhere. Did you crawl underneath and inspect every inch? They can get through very small openings. I had more of a problem with spiders. The biggest area for them to come in was right up the power cord. I cut a piece of sponge with a slit in it to go behind the power cord storage door. The inside box that the cord stores in had a huge hole where the cord comes through. There were places where propane lines and wiring had no caulk.
When they shrink wrap my pontoon boat they put in a few dryer sheets to keep mice away.