We have a Class A and DH always says that if we win the lottery, that's what he's buying! (so I'm safe there)
I think they are called Class C's.
I don't think there is any advantage over Class A except maybe access to the engine (in the front vs rear diesel pusher). One disadvantage is that they may not ride as well as a Class A. I also don't like the area near the cockpit as there is usually a hump and you need to duck if there is a bed or overhead cabinets.
Don't think mpg is any better since there are so many factors to mpg.
Try www.rvt.com (RV Trader) and search for Class C's. Dynamax is one brand. Powerhouse is another.