Greetings & Welcome!
The Dodge 318 drivetrain, prior to the implementation of smog devices in 1975-76 ish, have been the most reliable vans and motorhomes I have ever owned, and also got the best gas mileage. Beginning with the smog devices, the reliability went downhill, maintenance went up, and gas mileage went down. The slant 6 225ci engine was good, IF it had the cast iron block (some were aluminum), but it was underpowered for motohomes. 1976 Dodge drivetrain motorhomes were the sweet spot for me.
The 1987-1990 Dodge Grand Caravans & Plymouth Grand Voyagers, with the Mitsubishi 3.0L V6 engine and TorqueFlite automatic transmission was possibly the best minivan ever sold in America. Great, reliable, FWD drivetrain that included great handling in all weather conditions.
With Fords, the pre-smog, 1976 and older with the 390 engine was the sweet spot.
For Chevy/GM, the carbureted 283, 305, and 327 engines were pretty reliable. Anything bigger not so much. The biggest problem with Chevy/GM vehicles newer than the 60’s is the lack of quality and high maintenance costs. Vans had roof leak problems, and motohomes had overheating, transmission, differential, and brake problems, even IF they had one of the good engines. The gas mileage sucked on the newer 350 and up engines, and when smog controls went into effect it made everything just that much worse. I read somewhere that the last good chevy’s/GM’s rolled of the assembly line in 1964 with the single exception being the classic GMC Motorhome in the 70’s.
Leaks are always problematic. You want to make sure you have no current leaks and no moldy/mildewy smell inside. I don’t worry about older repaired leaks provided they were fixed, even if telltale signs remain, but no foul smells. After the drivetrain/driveability aspect, leaks are my biggest worry. Double check to make sure the tires aren’t outdated, as they often are on motorhomes. They can look brand new, but be very dangerous to drive on. If in doubt, buy new properly rated ones. Safety is important.
Be wary of propane leaks, they can kill you. Propane, solar, owner modified wiring, and more recently lithium batteries are among the top causes of RV fires, explosions, and deaths. Often RV repair people and shops aren’t even competent with these problems. BEWARE!
Everything else is pretty easily fixable, and relatively cheap.
"I can live like a king because I work like a dog." ~ An anonymous vandweller