wind

We purchased a B+ because we wanted something narrower, lower, easier to drive, and more fuel efficient. The B+ did satisfy three of the four items we were looking for.

We have a Winnebago Aspect 26A. It is on a E450 chassis. It is easy to drive, tracks well and not effected a lot by wind/trucks. I also like that it is narrower so its easier to drive in traffic.

However, the fuel economy was not its strong suit. Our previous 23ft Class C on the Chevy 3500 chassis with the Chevy 8.1 liter engine would get 9 to 9.5 mpg driving just under 60 mph. The Ford based Aspect E450 struggles to get 9 mpg driving the same speed.

The 23D Aspect on the E350 chassis seems to get 9.5 plus. I believe the difference is gearing and lighter weight. Our empty Aspect 26A weights as much as our Jayco 23RK did fully loaded and the Chevy engine appeared to be a little more fuel effecient.

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.

If you're lucky, it might be worth a 1/2 mile per gallon difference. If it's fuel consumption that you're concerned about, you probably need to decide if you can live with 8-10 mpg; if you can't, you won't be happy with an RV.

I think you meant the "big overhang on the **C**" and from what I can tell the C is about the LEAST aerodynamic face you can put on the road. There are not all that many small Class A's but the few I've checked on seem to get a little better fuel mileage than a comparable C.
But as a group, the "B+" units tend to be lower, even narrower, and usually shorter, than the full "C" which reduces Frontal Area as well as Weight.
Additionally, if the units in question are on Ford chassis, most recent C's are on E450 and the "B+" is likely on E350. The 450 is heavier, sits a little higher, and has a steeper axle ratio. So if the "B+" is on the lighter chassis, it should be a little more efficient.
Actually, in motorhomes, I think the appropriate way to say "efficient" would be to say "less inefficient."