Most are aftermarket conversions or built by specialty, low-volume manufactures.
Quigley is perhaps the best-known converter; they'll convert an E-series based motorhome (that meets certain base requirements) for about $14,000 according to their web site, if I'm reading it properly. They do a few other chassis as well.
Tiger is one fairly well-known maker of 4x4 class C's, built on truck chassis rather than the more common van chassis. From what I understand, they're nice and well-built, but not particularly spacious nor inexpensive.
Even with four wheel drive, an average class C is hardly a superior off-road vehicle: it is tall, wide, has a high center of gravity, poor departure angle, and poor turning radius. While there certainly are occasional situations where four wheel drive on a motorhome is useful, they aren't super common. A truck camper on a four wheel drive pickup often makes a better off-road camper, at least for people who aren't made of money. That being said, I've taken my quite ordinary class C over some pretty rough roads without any real difficulties besides bouncing around, and on the occasion I've had to do some driving in a bit of a snowstorm it behaved reasonably--but I deemed it wiser to rest up and wait things out when I got to a good place to stop and do that.