Davydd explained it well.
Our Leisure brochure (and the guy we bought from) reported the mpg as 22 for our 2500 Sprinter van, but I was skeptical. We've kept records at each fill-up. In 26,000 miles and almost three years since we bought the van (used, not new), our average diesel mpg has been a little better than 23.
(Our best so far was 25+ last winter on flat stretches of highway, minimal stops and starts, at conservative speeds using cruise control.)
A new Sprinter van would be different. Some changes were made to 2nd generation Sprinters in the US market starting in 2007 and resulted in a reduction in mpg. Some Sprinter converters' RVs now are the 3500 weight class and have dual rear wheels.
Of course, diesel costs more than gas these days in the US.
UPDATE: Getting 25.8 on our current trip based on first three fill ups. Really surprised because the first few days were in the Great Smokey Mtn NP area, lots of climbing.