I had a problem about two years ago. They not only got inside the FW, they chewed the wiring for the brakes and lights. It cost me almost 1400 to have everything repaired.
I had the RV shop put rat poison pellets up inside the channels where the wiring is run.
I purchased two ultrasonic pest repellants that seem to work very well. I put one in the back of the RV and the other is in one of the front compartments. I haven't seen any rodent droppings in the RV since I started using them.
I sealed around all the plumbing with steel wool to keep them from getting back inside.