Another one to look for used would be the Xplorer built by Frank Industries around 2004-2005. This was on a F-450 (or maybe F-550, whichever was just under 18,000 pounds GVWR) cab-chassis but wasn't sold as a Super C because it was a small house, for about 26-feet overall length, thus about the size of a 22-24 foot conventional C.
The Xplorer wasn't terribly expensive, about 30% more than a comparable Bigfoot, thus about twice the price of the cheapest conventionally constructed C's on the Chevy 3500 chassis. Roughly $100,000 in 2005, before paying the premium for a diesel and the 4x4 conversion if you must have that.
Building for excursion pushes the price up. It is not just that the chassis is a lot more expensive than E-350/450 with the RV package, the house has to be built differently as well, to stay intact running on uneven surfaces.
Visit one of the Thor or Forest River C motorhome factories to see how the box is tacked together to meet the low price points expected in this RV market segment. It is not at all like the auto industry, where we've come to expect Toyota's Yaris to have the same basic structural integrity as their Lexus. There can be big differences in RV structures for different price points.
If I were going 4x4 on a van-chassis C, I would want it built at least to the level of Phoenix Cruiser or a Bigfoot, and I don't think anyone besides Host and Tiger is building something like the old Chinook, which was popular for conversion to off-road use. But if I really wanted a motorhome for off-road, it would be a van conversion (Class B) to get the structural integrity of the van, rather than try to build a strong enough house-in-a-box to put on a light truck chassis.