Tips for keeping mice out?

I've heard of a couple: moth balls (not too healthy for humans, though!), or the strongest scented fabric softener sheets. Right now I've got 4 electronic devices plugged in that emit a sound that the critters don't like and I think that is working. Plus moth balls in the basement compartments and the sheets scattered throughout inside the coach. I've seen products at RV stores, like "botanical rodent repellant".

Do any of you have some favorite products or recommendations?