Aren’t all van based RVs considered Class B? I thought that was the definition. And wouldn’t they be lighter and get better fuel economy versus the Class C’s that are based on a van chassis?
The definition is that class C is constructed on a cutaway chassis, constructing new "RV walls" at the RV factory, and class B uses the original "vehicle walls".
A class B will get generally better fuel economy than a class C, because it's more aerodynamic, narrower and not as tall. Of course, a class B will have less interior room.