The difference in fuel economy does not tell the true story. Given how much higher priced diesel is than gasoline, (lets say $2.43 gas/$3.05 diesel) and a fuel mileage at the same speed, of 11 mpg for the V10 vs 15 mpg for the Sprinter, the cost per mile difference is only about TWO CENTS per mile.
My older WBO View with the five cylinder diesel get better fuel economy at the cost of performance, vs the newer V6 powered Sprinter chassis motorhomes. Over the past 10K miles (almost exactly) since I purchased it, I have averaged 17.09 mpg, but a good bit of that is lower speed 55mph state highway driving and some local driving. On my 3700 mile trip to the Black Hills and back the mileage varied depending on terrain, wind, etc, but generally the interstate driving at 63-66 mph netted me 16.5 mpg as an average. (its all in a spreadsheet).
All of that being said, If I were looking to buy a motorhome today, I would be looking at a ten year old Class C on a Ford E450 chassis (4 inch wider track than an E350) and way wider track than a Sprinter, and a whole bunch of other reasons. This is my first RV and I went into it saying that I didn't want to get stuck with something that got horrible gas mileage in case gas went out of $ight again.
Truth is, I want to get away from a MH altogether and get a decent tow vehicle (2500 something.... diesel) and a 22-25 ft travel trailer. The Unicorns just haven't brought it to me yet.
Charles