I saw a few custom made class B units years ago. This was just before FedEx had a fleet 4x4 vans made. There is a huge host of hurdles. The house has to be sturdy enough to handle the rigors of 4x4. The axle has to be rated for the weight. None of the typical van chassis used for "C" chassis come in 4x4.
It would seem more unlikely in a "super C" chassis, where pickups are more common. Even then, these chassis is not used in 4x4 form for RVs that I have seen.
Here is a link with a DIY build.
http://www.thedieselstop.com/forums/f202/justdifferentials-f450-4x4-rv-build-thread-305976/IMHO i'd skip the 4x4. I'd either downsize to a pickup camper with 4x4, or look for a locking differential for an axle in an axailable class C. With a tire pressure adjustment, a locking diff, and DRW, if you can't get in and out of a site a class "C" doesn't belong there.