I've not had this problem but I have read of campers who find that over the winter, mice have moved into their campers. Ick!
Emptying the cupboards of food and paper products doesn't always mean it will keep mice out..they like a nice, cozy place to spend the winter. and if there's a food source outside, they'll bring it in. They even will take up residence in the air filter compartment or cabin filter in your car or truck.
We have had problems with them living in the insulation in our household pump house (we are on a well and the water tanks/pumps are in a separate shed).
What we do is keep the interior of the camper lit up. I've put a string of LED Christmas
lights on the floor of our
Casita, running from end to end and also tucked into the hot water heater compartment.. I also make sure the AC power cable is up off the ground, so that it doesn't give mice any way to climb up into the camper.
Not that everyone can do this, but we also have a nest box for barn owls near the house. I love barn owls and they love to eat mice, so it's a win win situation.