There are few if any short diesel pushers. A large part of this, as I understand it, is because the amount of space in back of the rear wheels required for the engine and transmission would force the rear wheels to be too far forward on a short chassis.
I know you weren't really interested, but a good few class C's are somewhat better in terms of ground clearance than most class A's. They still are far from exceptional by and large, the departure angle especially being a poor point thanks to the long overhang in back. Class C's based on a van chassis also often have a pretty horridly large minimum turning radius.
Perhaps your best bet would be to convert a smallish school bus. They are generally designed to be able to go over less than perfect roads, and in rural areas often do travel over dirt and gravel roads. Making an RV conversion is a whole lot of work, of course.