I'd go the other way. Once a cutaway is heavier duty than you would typically use to to create an ambulance (in the short wheel base version), I'd call it a truck conversion, then just specify what class truck as a sub category. The problem is all truck conversions I have seen are also cutaways, and as currently defined, any cutaway is a Class C. If you preserve the truck cab, you have a C.
The real problem is the snobbery used try and distinguish RV types and only allow certain ones. It is always going to let in some lower units and exclude some higher units.
Jim