I would not be too worried about mice in a RV. Just throw a cat w/litter box in there for a week and the problem will disappear. You could also set a pail mouse trap inside to catch multi mice fast. I would half fill the pail with water or used oil in the winter to drown the mice (oil would not freeze).
At a min, I would check the RV out really close to verify everything works, including checking the air flow through the duct work. Mouse droppings would also make a good negotiating point to lower the price.
We lived in the country and mice would always invade our house and shop during the fall months before the onset of winter.