It has been said they use the dryer sheets as bedding and the mothballs to play soccer. Could be.
Many of us who have a feed room at our horse barns swear that leaving ceder chips on the floor deters mice. I haven't really decided if it actually works or not, but we do it. So in that vein, I keep a couple of socks full of ceder chips in the camper. One in the cabinet under the galley, and the other in the front storage compartment.
But I'm with you, the most important thing is to keep the trailer sealed up. I have not had any mice in this trailer. Could be the cedar chips work ? But then, I have not had any bears, lions or elephants in here either, so who's to say ???? :) ;)