Black's Legal dictionary doesn't contain a definition of Class A motorhome, but there is a generally accepted definition of class A. Here's one such definition from changin gears who quoted Gulf Stream.
Definition
Photo courtesy of Gulf Stream Coach
The Class A motor home is a recreational vehicle built on a stripped truck chassis where the driving compartment is an integral part of the RV interior. Class A motor homes look like busses.
Unlike the Class C motor home, the chassis of a Class A does not come from the factory with a driving compartment (or cab). The chassis includes the following major components: frame, engine, fuel tank, axles, drive train, wheels, tires, steering column, engine and braking controls, and dashboard instruments.
Other terms used for Class A motor home:
Coach
Motor coach
Sub-Types
The Class A motor home has one sub-type:
Bus Conversion
The name gives it away—a bus conversion is a commercial passenger bus that has been converted into an RV. Some bus conversions are easy to spot because their previous skin is mostly intact. Often these look like a Greyhound bus. Some bus conversions undergo major remodeling inside and out, and are more difficult to see their origin. One of the most used platforms for luxury conversions is the Prevost bus.