Welcome to the forum Mr. Wizard!
Look like you could expect from 9mpg to 14mpg according to this link:
It depends on the motor size. An RV conversion will get a little less because of the extra height and weight.
As with any older vehicle:
Tires (if more than 5 years old), fluids, filters, belts, hoses, plugs etc. might all need changing. Brakes should be checked.
If it has really low mileage it can indicate:
1. the odometer has gone around and it starting again
2. the previous owner (owners) really didnít like to drive it.
Hopefully some other members here will have some advice for you.