FWIW - The new 4 cyl Mercedes Sprinter has essentially the same HP and torque as the old 5 cyl model.
The Sportsmobile website lists the mpg reported by their owners as:
Ford V-8 15 mpg
Ford V-10 12 mpg
Chevy V-8 14 mpg
Sprinter diesel in short body 22 mpg
Sprinter diesel in long body 20 mpg
I completely agree with Davydd's comment that the interior layout of the van is far more important than a couple of bucks +/- in fuel costs. B's have a limited amount of space and it is critical to get a layout that fits your particular needs.