Check around your fire wall and up under the dash board. There are multiple holes for electrical harness passage. Before you park shift the dash board air to AC/recirculate just incase there's an opening for fresh air they can get thru. Power cable from the generator is another area. Check you storage compartments around any light fixtures for gaps also.
If you can stick you pinky in a mouse can enter. As mentioned
Great Stuff expanding spray foam that has a bittering agent to prevent crewing. Regular steel wool works but will rust with time. Stainless steel pot scrubbers will out last the coach and are best.