Hi Bob and welcome.
A class B is a van that is converted to a camper. A few years ago the folks doing the conversion started removing the body of the van from the drivers seat back and replacing it with a wider body.
A class C is built on a cut-away van chassis. The cab part with the engine, frame and everything that is needed to drive it comes from the truck manufacture. If you go to a RV factory you will see them drive this into the plant, where they will build the floor, walls and roof and place it on the chassis. They build a section that goes over the cab, where they normaly place a bed or a TV. For many years Trail Wagons made an RV called a Chinook on a cut-away van but with the over the cab section lined up with the windshield to improve the air flow. Chinook (they called it a class C) was a high end RV and the cost got way out of hand for most people. Along comes a trailer manufacture named Trail lite and they start copying the design and floor plan of the Chinook and making a lighter and less expensive motorhome. In order to make theirs different, they called it a class B+. For many years after this the argument about what is a class B and what is a class C went on and no one knew where to place this class B+. Was it a class B, not really. Was it a class C, some say a Class C is built on a cut-away with the overhead bed, some even said that it had to have dual rear wheels. So this new thing did not fit either class, lets just leave it as a class B+. It think it is really a class C minus.. Minus the overhead bed.
