The drivetrain is almost identical to what was used in a 1st generation Toronado, 455 V8, THM425 transaxle. It’s the original color, a very pale beige, almost white. Has a stripe down the middle of the side in bronze, rust and orange. It’s the Royale model. The front and rear caps as well as the lower sides are not quite fiberglass and not quite plastic, it’s the same material used in Corvettes of that period, called SMC, which is sheet molded compound. The upper body panels and roof are aircraft grade aluminum, on an aluminum cage. The rear tandem wheels are on an air suspension. It drives and rides similar to a full size Cadillac from that time period. If everything is aligned correctly you can drive it with your finger.
Wow, what you wrote sounds like my relative, who told me almost exactly the same thing. If you ever have a question about it, PM me, you should talk to him. He was in charge of a lot of things concerning the GMC motorhomes at that time and lectured on it for trainees or something like that.
Good luck and God bless with your GMC baby.