First check for water and rodent damage. Find either, run. Drivetrains are secondary to damage like that.
As Ford versus Chevy, both have their fans, both have their detractors. Can't really go wrong either way. Fords are more common, and other than the two piece spark plug issue, have been pretty reliable.
If I were buying a 2002 E-450 chassis, after making sure it is in decent condition (if a "C", checking the cabover for hidden leaks, etc.), here is what I'd probably do next:
1: Replace the coils and plugs. If the auto shop doesn't know what a Lisle tool or Time-Serts are, I'll find one that does.
2: Reseal all seams. Best way is the rvroof.com epoxy roof spray. Second best is Rhino's similar offering. Third best is going to town with Eternabond, and dead last is Dicor.
3: Change the oil in the generator with an oil extraction pump, mainly so I get the sludge in the pan as well. Newer Onans use splash lube, so go with synthetic because the oil gets an extreme workout with this mechanism.
4: Have the generator checked out by someone who can monitor voltages and load tests. Likely in an '02, the carb needs work or replacing. The coils may be corroded and may not give good voltage under load.
5: Replacement tires, brakes, and batteries.
6: Engine oil, ATF, grease, radiator, etc. All fluids changed.
7: Sanitize the tanks thoroughly.
8: Have all bedding, carpets, and such steam cleaned/disinfected. Ideally heated to over 160 degrees for a few hours so if there are any bedbugs or whatnot, they will be quite dead.
9: Probably chuck the mattress unless it is an oddball shape (corner queen).
10: Check the RV converter. If a single stage, chuck it and get a multiple stage one that won't roast your jars.
11: Have the vehicle rekeyed, PATS reset and only your keys entered into it. This way, if someone who owned it previously had a key to your rig, it won't open the vehicle, much less start the engine. Chevies, this is easy. Fords require a stop to the dealer for a reprogramming.
There are a lot of good used rigs out there, but it takes some good looking to find the genuine gems versus one that is one stop from the city dump due to water damage.