I actually get 10-10.5 mpg. I commute in my vehicle (and use it for an office). To me that means driving with commuting traffic at those speeds, so realize that I am driving 70+ mph alternating with urban traffic (and road conditions). In that environment, with about 1 hour of generator time per tank in the winter months, I have been in the range above. The only trip I made was to Colorado/Arizona and drove at least as fast as I would have in a car and got about 10 mpg. I suspect that if one would drive 60-65 mph, 12 is reachable. Now, the 8.1 engine is an improved version of the 454, and I suspect that it gets better mileage than the 454 in spite of having substantially more power. The story about the 6L engine is similar. I am frankly not as familiar with that engine, except that I know it is a "small block" engine. I believe that it has modern valve/cam technology, but is basically the same design as the small block chevy engines of old. The 5.7L engine would get about 12, and I suspect that the 6.0 would get about 12-13 in similar driving to what I do. As much as I LOVE that big block engine, I would have gotten the 6.0L engine if it were available last year. It has almost as much horsepower as the Ford 6.8, and I suspect that it has enough extra power for most towing needs anyone could imagine. If your plans are to tow over 5,000 lbs routinely (which would require a different hitch), maybe the Ford is a better choice. My suggestion if you are trying to decide, is to drive both drivetrains, and compare the stability, room, smoothness, and power on the road. Then add about 1-2 mpg, and see how your decision feels.
It would be difficult to get much better information in comparison from other drivers, unless they have driven both drivetrains in a similar manner (as far as mileage goes), and you deserve to drive both for your own decision.
Dan
2006 Lexington 235S; 6.8L Ford E450,
2005 30RLS Outback Sydney Edition,
2007 Jeep Compass
2009 Mini Cooper S