Sportsmobile has 3 factories in Texas, California, and Indiana. The California location specializes in all out off road vans with a 4x4 conversion package and raised bodies. These are more than capable of travel on bad roads or no roads at all but are fairly pricey.
All three locations make custom built B vans from a variety of van models - you design the layout you want and they will build it. They will certainly build vans that have nothing hung under the van if that's what you want. Mercedes also imports 4x4 Sprinters that they work on. For bad roads, you might want to consider the shorter wheelbase models.
Other non van options include truck campers and the Tiger Provan which is actually a class C built on a pickup body.
Fairly simple truck campers with a popup conversion are made but should be based on a 3/4 ton body. Truck campers that match the amenities of class B's generally require a 1 ton truck usually with dual rear wheels. They are very tall and very wide.
