thinking about getting 1982 B250 van

Realistically that van would probably get about 15 mpg unloaded. The OD833 wont like towing 3000 lbs in overdrive unless it has 4.10 gears out back, you"ll probably have to use 3rd gear while towing which will probably put you at around 8-10 mpg when hauling a 3000 lb trailer.