Yep, keeping the mice out in the first place is best. I've either been really lucky or what, but I have mice/rats all around my property and have more problems with them around my house (they seem to like chewing on my video camera wires now!)
But, have never had them in my 2 different TT's I've had over the last 19 years.
As far as what I keep in the rig over the winter, I take out all perishable food items. I've left some canned foods in there, but will rotate them every camping season.
I don't live in a really 'cold' area where it's freezing for months at a time, so I guess that would make a difference too..
I guess in the end, if you have to just hitch up and go and have not had a chance to stock the rig.. Carry a CC and go shopping along the way! :)
Good luck! Mitch
