Yep, I tried the "At max possible speed" before. I have also towed at 50-55 before. I now tow at best fuel economy for my truck (when safely possible).
With the 5ver loaded heavy, driving at 63 mph indicated, I get 12.4 to 12.5 hand calculated. (Display exaggerates by +2 mpg)
At 55 indicated, I get the same as driving at 72 mph indicated, 9.8 mpg hand calculated.
I proved this myself towing from Houston to Chicago. First trip I was in a hurry to get there and set up. Job was starting soon. I had to buy fuel in Illinois (ouch).
Fast forward three years later and the job is over. I first rode my motorcycle home. Rode like a mad man- 80-85 mph the whole way. Left Chicago at 5 a.m.- made Texarkana at 7 p.m. Funny thing though, I passed the same semi's multiple times that day on I-55. I kept stopping for fuel. Left Texarkana the next morning at 7 a.m. and was home in Houston by noon. Whew, glad that was over and I did not become a statistic.
Anyway's, I fly back to Chicago, hitch up the trailer and decided to go for max economy. I was on the road by 6:30 a.m. and stopped for the night just east of Texarkana at 9:30 pm. Up and back on the road by 8 a.m., finally made Houston at 3 p.m. (Several wrecks closed down the highway periodically that day. End result-I made the trip on a single fillup (109 gallon/1376 miles). No highway diesel markups for me and I enjoyed the trip without feeling stressed.
This works for me. YMMV