Yup, they're based on the GM Medium Duty Truck platform which was sold as a GMC Topkick or Chevrolet Kodiak chassis.
Couple of different manufacturers used them, Four Winds, Chateau, Dutchman. They're all "Super C" configuration (large class C) ranging in length from about 30' to as much as 40'.
The Jayco Seneca is the one I'm familiar with, which was made from 2005 through the 2010 model year (GM ceased production of the MDT in July 2009).
The chassis is quite awesome-- available as an 8.1 gas or 6.6 Duramax diesel, and is capable of pulling 8,000-10,000lbs in most configurations. Most of them were available with an Allison transmission. Up front, it's a mish-mash of GM parts. The cab itself is derived from the G-series vans and shares many parts (glass, doors, dash, interior) but the MDT has its own fiberglass hood that opens forward, like a Corvette. Most of them are built on the C5500 variant of the chassis, heavy duty steering, front and rear discs. Most of them came with 19.5" Alcoas fronts, and rear duallys. They come with 80 gallon fuel tanks.
Fresh, black, and gray water tanks are especially generous in the Seneca series for any Class C.
Drawbacks are...it's built like a truck, and that affects handling and ride. Leaf springs and a solid beam axle up front, so there's lots of jounce and sideways movement over bumps. A lot of people add a heavier swaybar up front and Koni shocks are apparently the go-to, although there are lots of good options.
Caveats are...04-05 the 6.6 Duramax diesel had a high number of injector pump failures. The later '05 and '06 received the updated LLY engine (with more power) which is basically the famed "LBZ" that were well known starting in the '06 Silverados and other light duty pickups. The Kodiak never got the "LBZ", it was forever an LLY, tuned for heavier applications.
I believe most of the 5500's came with the 5.13 Dana S130 rear axle good for 13,5.
'04-'05 had the Allison 1000, which was configured as a 5-speed.
'05-06 was the beginning of the Allison 1000 that was configured as a 6-speed. Older 5-speeds can be converted to 6-speed with relatively minor updates, as the shift is electronically controlled and done via clutch packs. The TCM receives an update which configures another permutation of clutch packs which provides an effective 6th gear.
Lots of reading on the GM MDT platform and duramax over at dieselplace.com which is a good forum supporting the chassis side of these trucks.
Our Seneca 34SS gets 11-12mpg mostly freeway and not towing anything, but camp-ready.....
