You can buy brass wool (like steel wool only made of brass) at Amazon to plug ALL the openings under the trailer. The advantage to brass is it does not rust and the mice can not eat their way through it.
If there is now a major infestion of mice, an exterminator can be called in and seal the trailer and fire off a toxic device that will kill all the critters inside. Then after the set time, open it up to let all that gas out of the coach. Might have to endure the decomposition aroma for the first few days as the mice "dry" out.
WBCCI Life Member 5123, AIR 70341, 4CU, WD9EMC
TV - 2012 Dodge 2500 4x4 Cummins HO, automatic, Centramatics, Kelderman level ride airbag suspension, bed shell
2014 31' Classic model 30 twin beds, 50 amp service, 900 watt solar system, Centramatics, Dill TPMS, disc brakes, 16" tires & wheels