Good you will see for yourself the space you will give up.
I understand fuel economy to be roughly this for a B+.....and I do mean "Roughly"
E450-V10 9-10 mpg
E350-V10 1 mpg better than E450
E350-V8 2 mpg better than E450
Chevy 2 mpg better than E450
Sprinter diesel 5 mpg better than E450
Pro-Master gas (just came out) 4 mpg better than E450
Ford Transit diesel (not yet available) anticipated to be like the Sprinter
There are just a gob-full of influential factors so these are terribly rough numbers assuming the same "small" motor home is on each chassis. E450s get weighted down the worst so that 9-10 figure would naturally be lower for most true-life conditions.
A Sprinter diesel "B" van conversion will yield your best fuel economy, with a Pro-Master gas on it's heals.
Many people who can't take the poor fuel economy any longer, find themselves in a Sprinter B+ similar to
THIS. This particular one happens to be offered on a Sprinter diesel, an E350-V10, an E350-V8, and even the E450-V10 for those who want the max CCC & towing ability. Oh yes, it is even offered as an E350 & E450 4x4.