Springtime following our trailer's first winter, I found LOTS of evidence of mice (droppings, etc., no damage). Needless to say this did not thrill my wife (me neither), so since I keep plugged into shore power over winter, I plugged in an electronic sonic pest-repeller inside the Bambi which emits a sound wave inaudible to humans and pets but supposedly repels not only rodents, but insects as well. Darned if I haven't seen any evidence of mice this year yet.
Maybe too early to say for sure, but worth a try! HomeDepot, etc for about 10-15$.