It is generally accepted that RV's average 5000 miles per year. Opinions on mileage vary but some think that 70K+ miles is OK for the Ford E-450 drive
train. I think that you should have a used rig evaluated by a qualified truck mechanic and by an RV repair expert unless you really know what to look for. Some low mileage or high mileage rigs or older but stored indoors rigs might be a good buy, some not. With any used rig replace tires that are over 6 years old, have brake system checked/serviced and brake fluid replaced, all belts and hoses checked and replaced as needed. Buying an RV is like buying a cabin cruiser boat, not much like a car.