A vehicle this big needs a big motor to get it down the road. For that reason, GMC used a 455 cubic inch V8 from the Oldsmobile Toronado. In later years, the engine would be downsized to the 403. These engines were fueled with regular gasoline, which was stored in two 25 gallon tanks. At today's gas prices, that would be about $150 for a fill-up.
The majority of these motorhomes were 26 feet long, with about 10% being made just 23 feet long. About 13,000 were produced, with an estimated 8,000 - 9,000 still on the road today.
People who own a GMC Motorhome are often part of a very close community, especially on the internet. This community helps each other in repairing and restoring their Motorhomes. It's part of the reason why so many of these vehicles are still on the road.
While there are plenty of these Motorhomes out there, finding one that is for sale can be difficult. You can either wait around for a retired couple who owns one to keel over, or you can check the listings on eBay. Either option sounds fun to me.