We have a 2005 Itasca 36' with the Caterpillar 350, 6 speed Allison transmission and 36,000 total miles. Towing a VW Jetta at about 60-65 MPH we are averaging 7.6 MPG via the computer. We live in south Florida and most of our traveling is in our own state so, although hills are not an issue, wind is. For example, on one recent 3 day trip we got instantaneous and fairly consistent readings of 9.5 to 10.2 on flat ground going with a wind of approx 10 mph and about 7.3 to 7.8 going against it coming back the other way. As an example of how this compares with our previous MH which was a 34' HR Admiral with a Triton V10 we were getting about 6 to 6.5 mpg pulling the same dinghy. Previous to that one we had a 31' HR class C with the Triton V10 and the same toad which was getting 10 to 12 mpg. My driving patterns were consistent between the 3 so the comparison should be fairly valid.
Before the motorhomes, we were towing 2 different 5th wheels with two different Ford F-250 diesels. With the 7.3L we got about 13 to 15 mpg and 12 to 13 with the 6.0L traveling at approx 70 mph. There is another consideration, though, when using fuel consumption as a factor in choosing between gasoline and diesel power; the price of diesel fuel is much less stable than for gasoline. At one point some time ago diesel prices here hit $4.85/gal at our local dealer when regular grade gasoline was well over a dollar less than that.
I know I got carried away with my response but I hope this info might be useful to someone out there. We've been camping with our family pulling or driving something for over 35 yrs and in tents before that and, regardless of fuel consumption, for ride comfort and feel of reliability the pusher diesel, except for basement storage capacity, has been our favorite way to travel.