First question I would ask is how are you going to use it? Are you going to use it a lot or just occasionally? How much stuff do you plan on taking with you? How much room do you need? Do you plan to spend much time at a campground with it or do you plan to arrive, go to sleep and head out first thing in the morning? I would first be asking myself those type of questions.
For instance, if you plan to enjoy the campgrounds a good but, then you will want something with some room. If you plan to be out and about mostly, then all you will care mostly about is a bed and bathroom. The class b market is a wide range. Form nothing more than a camper van, to nearly the size and look of a class c. Many have convertible beds while others have dedicated bedrooms. Same goes for bathroom. You can have a wet bath or a full bathroom. And kitchens can vary as well.