Dutchman did an EXCELLENT job of showing you how winterizing is done!! Look for your water heater bypass and follow Dutch's instructions, you can't miss!
As for the mice, we use dryer sheets, a whole box (small) strewn ALL over the interior of our coach. We have stored ours in a pole barn as well, I get some D Con boxes and place them around the outside on the floor of the trailer. So far NO mice doing it that way.
Good luck winterizing, if you can get somebody in the know to give you a hand, there won't be any surprises next spring then.
Dan