The problem with pricing a GMC is 2 fold. There are a ton of units out there with very high price tags on them. This has driven the market artificially high, even for ones that are not restored. The second problem which was eluded to earlier is that any motorhome, regardless of the collector factor will never be worth what someone has invested in it. This puts dollar values all over the place and makes it very hard to find a stable consistent why to come up with real numbers. Of course there is also the issue of what something is listed for, compared to what people will actually pay. This range is rather wide, especially for an older unit that has been redone.
As far as the over all big picture, as a general rule, GMCs were much better at traveling than they were at camping. You have a 26 foot unit, that is laid out light on the amenities, and heavy on the number of beds - which was a common philosophy in the 70s. To be able to sleep 6 in a 26 foot coach, you loose space in other areas. One of the biggest is the bathroom. Most GMCs have a wet bath, which means you shower over the top of the toilet. Unless you are looking at a rear bath model, or one that has been modified, you don't have a separate shower. (yes there was the Royal, but they are rare).
As far as the engine and drivetrain, the 455 was a very durable engine as long as you didn't run high RPMs too often. I knew of a Revcon owner (uses the same drivetrain) who had almost 300,000 on the original engine and drivetrain. It would still be running today, except for a propane leak that ended the coach's life. The Revcons are lighter, so they don't put the strain on the drivetrain, but even so, the 455 was very durable, as long as you didn't wind it out. One heavy wear point with a GMC is the front wheel bearings. This is a high maintenance item. Not the end of the world, but something you just build into your maintenance budget and schedule. Other than the front wheel bearings, I do not believe the rest of the drivetrain is prone to wear any faster than any other motorhome, however when it does wear, the cost to repair is about 1.5 X the cost of a rear wheel drive coach. If you paying a premium for the coach, you need to know the condition of the front end, as that should significantly affect the price.
My personal opinion is that you are paying a lot for the wow factor, and not necessarily for the actual value of the coach as a functional camper. Sure wow is cool, or otherwise people would not own or restore antique cars. As a coach, it is a little better built than many of the coaches in that era, but its not some magical super high end unit. Unless a very significant purpose is collector driven, as a camper, it has its limitations.
