A SD RV dealer came up with this and says it's how he keeps his inventory rodent-free (most of it sits outside on the Great Plains all winter).
Spearmint oil seems to repel the critters 99.9% of the time (he said). The only time he has problems is if he's taken a unit in trade that already had/has a mouse infestation; then it doesn't seem to work. Nor will it repel critters who have already taken up housekeeping.
The technique he employs is to saturate cotton balls and place one each in a small paper cup. Then he puts one or two cups in typical mouse incursion locations throughout the RV. (I didn't ask if it made the units Wrigley going down the road....
For those of us who store their bikes/cars/RV's et. al. in vulnerable locations for rodent attack, this is a great, pet friendly, focused way to keep them damage free.
I've used Downy dryer sheets in my bikes' saddlebags for years and never had any mouse problems. I think this year as the temperatures go down and the critters start looking for winter nesting sites, I'm going to give the Spearmint Oil a try in my garaged (barned) cars, JD, etc. If it keeps million$ of dollars worth of dealer RV's safe, it's worth a shot.
You can find any number of vendors online, including Amazon.