We have an 08 Winnebago View and tow an 08 MINI Clubman.
Average mpg as a notowbago is around 17, with the Clubman dinghy it's around 13 mpg. Not underpowered at all, but then, we don't overload it either. ;)
Hubby just added supplemental brakes to the clubbie because of legal requirements, not because it needed it.
We have previously owned one of the five cylinder engine RVs and used the same dinghy. No appreciable change in power but the mileage was better, about 18 notow and 14 with tow.
It is not a B+, it is a class C. This is per Winnebago who should be the subject matter expert.
A class B is built within a van shell, with possible roof mods. A class C is built on a cutaway van, with the entire box being added.