I've had a couple of these and the back door seals leaked on both. Never was sure if it was the seal or of the doors weren't latching close enough. The scheme where one door latches to the body and the second door latches to the first door leaves something to be desired, especially with the weight of a spare tire carrier added. Let me know if
your shop comes up with a good solution.
Check carefully for leaks and rust in the windshield and front cowl area. A windshield leak that drips onto the parking brake pedal is pretty common. Any openings cut for RV equipment are also prime locations for rust
to start. With your van's history I wouldn't expect a lot of rust, tho, mostly just leaks from dried-out seals.
1990 E-250 Sportsmobile w/ penthouse top, converted when new by SMB Texas.