Answer:
In simplest terms, the Class A motorhome uses a single body with a flat nose - they're often built on bus chassis, while a Class B uses a second body which is attached to the cab, and the cab has a hood which extends out from the front. Even a Class 8 truck (such as the Freightliner Columbia or Volvo VNL series) which is made into an RV is still a Class B motorhome.