I think a lot of B class buyers are first time buyers. They typically find out they want a real shower, toilet, bed, bigger tanks, closet and storage. The gas mileage is better but you sure give up a lot and some of them cost twice as much as a C class.
I've driven from Tampa to Cincinnati in 18 hours with our C class which is 1025 miles. That's with walking our dog, getting a bite to eat, stretching my legs. Those Monster Coffee power drinks give you super human strength but then I want to sleep. LOL