There has been lots of Class B MPG discussions in this forum. It depends on what kind of B you want as they vary in fuels (diesel/gas) and mpg. Regardless they are all considerably better than any C. They probably range from a low of 13 mpg to 18 mpg with gas engines in overall average from what I've read in this forum.
For diesel my 2011 Sprinter Great West Van Legend has an overall diesel consumption after 50,000 miles of 18.6 mpg average. Highest I recorded on a fill was 20.4. My former 2005 Sprinter Pleasure-way Plateau had a 22.5 mpg average over 61,000 miles. The highest was about 25. You can achieve those highs mentioned driving on flat roads at 55 mph with no head winds.