I had similar to Dittmer on the 2350. But on the 2551 and 2552 it was only slightly less. Actual results are:
Not towing = 9.8.
Towing = 9.5 (3,000 lb. car).
The above is what we averaged on the last 7,000 mile trip from PA to OH, Elkhart, IA, NE, Denver, Durango, NM, Salt Lake City, Yellowstone, SD, MN, WI, IL, KY, WV, VA, MD, etc. Many of the miles were mountainous in 3rd gear (over Logan Pass to Bear Lake, for one instance).
Actually, I think towing smoothed the airflow at the rear and allowed considerably better milage on the flats (we prefer to stay off Interstates).
Regarding an F-550:
I really like that idea and might consider a Diesel 2552 Phoenix if it were available on that chassis. It'd sure be easier to not contort around the engine cover when getting to the rear and not have to listen to the dreaded Powerstroke lullaby when travelling.