I have logged differences in towing mileage when running 87 octane fuel vs midgrade 89 octane. When towing I run nothing less than 89 octane. Blended fuels such as E-10 have also resulted in lower mileage. For example, towing my 18 foot boat with gear, or roughly 2800 lbs, I do about 1-2 mpg better with 89 vs 87 octane non blended fuels. I generally see from 11-14 mpg towing this load.
The return trip from Tucson to Phoenix with my load yeilded better mpg that the trip down on Wednesday night. Traffic flow was much better, and I was able to maintain a solid 75 mph for the majority of the trip. 9.3 mpg towing 9,000 lbs of truck on flatbed trailer at 75 mph in 4th gear. I think I might've been going a bit fast, and the mileage suffered accordingly. I've done this leg at 70 mph and maintained just over 10 mpg. My 99 F-150 5.4 might've gotten 7 mpg on the same trip at 70 mph.
It was entertaining seeing the looks on the faces of the guys in 3/4 and 1 ton trucks going 60 or 65 with less weight than I had. I don't know if it was fear because they thought I was dumb, or awe that they were being blown past by a 1/2 ton pulling more weight. One guy in a Dodge 3500 DRW kept passing me, only to slow down and let me pass him. When I'd roll past I'd see the black cloud of a diesel under hard acceleration as he went to pass me again. I think I somehow assaulted his manhood, but I just hit the next rest area to get away from him.