oregonct, you do not save much - just a little when you compare diesel vs gas. Gas motorhomes are cheaper, diesel fuel is more expensive and so is maintenance. I average now about 14,5 MPG, (it went up by 1 breaking about 10k) it all depends on driving style and the a/c is a big factor! I got as high as 17 sightseeing through the Colorado mountains and as low as 13.5 in South Florida with the a/c running continuously...
In the long run - 100000 miles or 10 years for me - I estimate a savings of about 5-7000$ with today's fuel prices. (2.99gas /3.59diesel = Naples today)
More important for me is driving comfort. Take a seat in a sprinter, note your leg room from the driver and passenger side and get out of your seat to the back of the MH and do the same in a Ford or Chevy and compare.
I owned a E550 (2005 Jayco Granite Ridge) before - what a difference!
Cruising down the highway is another point - the diesel is more quiet and the downshifts are so smooth compared to my Ford!
It is not only fuel and mileage, compare and test drive!
Just my 2 cents.