There's real problems on the GMC with frame rot and fatigue. Agreed about the design, it was exciting when they came out (1974) I remember seeing one in AZ that summer in the White Mountains. However, internal storage space, etc, made more than 2-pax into a problem considering V8 fuel mileage (for use as a place to live).
The FMC was then, and is now, a better choice (fuel mileage about same) but internal capacities (storage/tanks/space) is better. It was considerably upmarket as well, so build quality is great (FMC is a military contractor, Bradley Fighting Vehicle). Fully independent suspension, Chrysler 440 V8 pusher configuration, internal A/C, low clearances ground and roof. Plus there are Duramax diesel conversions to get 14-16 mpg.
The history of the FMC Motorcoach
The GMC was really about a bunch of Brooklynites running south on I-95 to Florida a few times a year. For that they were great. There's a guy in FL who specializes in resto work and upgrades that are nicely done. A hot Joe Mondello Olds 455 -- with disc brakes -- is the way to go, IMO, when transporting 4 people with or so to a resort hotel or a hunting/fishing trip, etc.