Found these definitions on an RV website... .
The Class B motorhome, also known as Van Camper, is a panel-type truck that has been customized to include two of the following conveniences: sleeping, kitchen and toilet facilities, also 110-volt electrical hook-up, fresh water storage, city water hook-up and a top extension to provide more head room.
The Class C motorhome (also known as Mini Motor Home), sometimes referred to as a Mini-motorhome, provides the conveniences of a larger motorhome in a scaled-down version and at a lower price.
They range in size from 10,000 to 12,000 pound gross vehicle weight, from 20 to 25 feet in length and about 10 feet high. The Class C is built on an automotive manufactured van chassis with an attached cab section with access to the driver's compartment. Their driver compartment is similar to a van with a large box in the back. The passenger can move from the passenger seat to the back of the unit.
Size: 20 to 28 feet