RV sale pending, but we found evidence of mice...

I would say discuss this with the owner. You should be able to check everything before purchasing. Use the electrical, make sure the furnace and all appliances work, use the water systems and pump. See if you have any strange smells with the furnace running and it's warm in there. Refrigerator, water heater etc. If you are parked anywhere in the country, near fields or tall grass, chances are you will have them again. Try to close off any small openings with steel wool. They can't chew thru it.