The only real way is to stop them getting in - and I guess that is easier to do in a travel trailer than in a large motorhome.
Lots of crawling around underneath blocking up openings around pipes and cables, checking fit of seals, seams that have opened up a little etc. Might need to have a strong light shining around inside at night while checking outside. A mouse can get through a slot 1/4" high and 1" wide so no hole is too small to be ignored.
Mice I know find dryer sheets perfect for shredding up to use as a nest lining.
Lots and lots of poison baits laid underneath and in the vicinity will get the population down very low and reduce the chances of survivors making it inside.
Tony Lee - International Grey Nomad. Picasa Album
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RVs. USA - Airstream Cutter; in Australia - MC8 40' DIY Coach conversion & OKA 4x4 MH; in Germany - Hobby Class C; in S America - F350 with 2500 10.6 Bigfoot camper