Someday I hope to see the actual records of one of the people that averages out 7.5 to 8mpg towing a 4,000 pound vehicle. Over 6 years 40,000 miles and two Ford V10 Itasca MHs I have an average around 6.3mpg.
I have on rare occasion had a tank run 7.5 or better and I've had a few under 6mpg. I drive 65 most of the time and have found this better than too much slower. I do drive in NJ, PA, VA and the surrounding area so I am in hilly, not mountain country and certainly not flat plains.
I just did a 2200 mile RT to Georgia and back. Took 81 south and 95 back due to weather. return mileage was better as 95 is flatter, but not significantly.
6.3 MPG towing and relatively empty.