Another to keep in mind is, from what I have read, mice do not like to walk across "open" areas. I know this isn't possible for everyone, but if you store your RV on concrete or gravel it will help.
Also, make sure there is no food in there, that's mostly what they are after.
mcfarmall is dead on about looking for holes, look everywhere underneath and seal it up.
We stored our popup and now the 17Z on a concrete pad, have yet to see a mouse. We also rarely eat meals in the camper, so there isn't much for them to find food wise.
2013 Starcraft Travel Star 229TB
2011 Jayco X17Z(traded in)
2010 Jayco Jay Series 1207(traded for the 17Z)
2011 Chevrolet Silverado 2500HD Z71 CC 6.0L/3.73
Reese "Pro Series" 800lb Trunnion WDH w/sway control