Unusually low mileage is not always a bad sign. It would be wise to have any used rig checked over by a pro truck mechanic. The entire brake system should be checked and brake fluid changed, brake calipers should be checked for sticking. The engine cooling system and dash AC should be checked. Tires may be unsafe due to age and sitting parked. Run all appliances in all modes to verify they are working properly. Have owner/dealer run fridge overnight to verify that it is working. Run RV generator, roof AC and microwave at the same time.
How many times have you read a real "tale of woe" about a low-mileage used rig??? May be a lot of "hearsay" about this and other subjects.