It original posting question is very difficult to answer with accuracy.
There are different B+ design variations. Ours has a flat area within the cap just above the windshield for wind to catch, but does have angled transition walls behind the cab doors. The over-all height is quite low-profiled measuring 9'-10" to the top of the a/c unit.
Our 2007 E350-V10 shown below during 3 & 4 week vacations (4000-5000 miles per trip), averages a fairly consistent 9.5mpg when towing our Jeep Liberty and 10.5mpg when not towing anything. Of coarse open road numbers are better, but my figures are over-all trip averages with combined open-road, mountain, canyon, urban, etc.
Another equally important benefit to aerodynamics will be improved handling. Then there is the "integrity" benefit, better protection against water infiltration. If you don't need the extra bed, then I don't recommend considering a class-C. Of coarse there are always exceptions such as price, floor plan, etc. I comment only in general terms.

