SkiMore - If you haven't searched yet, several Class C makers build on a 4x4 chassis. Some are a little more capable off-road than others.
Chinook Baja
Host
Thor's Chateau and Four Winds
EarthRoamer
Tiger Adventure Vehicles
Sportsmobile
ActionCamper
Xplorer
Global Expedition Vehicles
It's also possible to get many 2WD chassis converted to 4WD, but it's spendy.
Here are some pics with ideas for off-road RVs -
ClickyAnd here's a link on this forum where SoCalDesertRider explains some advantages of putting a TC on a flatbed.
Clicky