Jim makes a good point about the Class B selection at many RV shows. Some have only one or two makers represented, unfortunately. I saw one brand at the first RV show we attended, and the interior turned me totally off class Bs for a long while until a much better one turned up for sale in my town.
It's useful to browse various manufacturers' websites, also websites for dealers close to you, and the site for a big outfit like PPL MotorHomes in Texas to get an idea of floor plans and prices. I ordered a lot of B brochures back in 2009 when we started looking.
As far as fuel mileage per gallon, you'll find a range in class Bs from about 12 mpg for gas engines up to 22 - 25 or so for some diesels. Obvious variables are how fast you drive, how much weight you are carrying, whether you tow something, etc.