Since you provided very little info about the condition of the moho other than apparently the engine starts and runs....
Before spending more than $100 on it. Assess whether it's a gem, a diamond in the rough, or a mushy leaky basket case.
If it's #2, proceed with caution, #3, hopefully you can find someone to haul it away or drive it away, or part out the drivetrain and stuff the rest in a dumpster/haul the chassis to a scrap yard.
Another word of caution, if you need to take it to the "mechanic" for a "once over", an almost 40 year old van parked at the cabin for 10 years may not be financially viable for you, even if it's in "ok" shape.
I'd go to the camper armed with an air compressor, extension cord (to plug into the cabins power, a good battery, bottle of propane and a garden hose.
Make sure there's air in all the tires, trans fluid looks ok, engine is not low on oil and coolant is full and brakes don't go to the floor or don't stick. If all that checks out, fire it up and take it for a hot lap and see if it appears roadworthy.
If it's not, the rest of the RV doesn't really matter.
If it is and it drives good and doesn't come back with a stuck brake or fluid pouring out or overheating, move on to the house.
Put a battery in it and see if all the lights and water pump work.
Fill the water tank, see if it pumps and check for leaks.
Fire up the fridge, stove and furnace.
Check for rain leaks (presuming there isn't obvious water damage. If there is assess that before worrying about the test drive).
After you've done all that and compiled a list of everything that does or will need work, put a $ figure to each item, however you'd handle fixing that item. Add up the numbers, multiply by 1.5 if it sounds reasonable to you. See if that number sounds reasonable and then decide whether to call your mechanic and tire shop or a scrap yard.
Good luck!