Do not worry about it. Look at it this way. The dealer called and told you about the mice, not to make you feel bad, but to make you aware. Maybe he figured, if he told you, you could do something to prevent it happening again.
I don't quite understand how this dealer has never encountered mice and a zillion other problems in used RVs. I've seen moldy PUPs, leak stains, etc. on used lots.
When we made out last deal, they asked us if there was anything wrong with our trade, (class C). We, honestly, said no. Sure enough, when we got it up there, the 'check engine light' was on. We honestly did not know about it, and felt really sheepish. It turned out to be not a big problem, but stuff happens. Go camping!