68 MPH will take the mileage and put it in the 6.5 MPG range. But sometimes that is what is required to get the trip over and back home. In other words if you need to travel 650 miles, then 10 hours at 70 will get it down with a break for fuel sometime in the middle, while driving at 55 you might end up taking a extra day to cross the country, and that means another day off work or about $250 in lost wages (even if paid time off).
So while driving 55 - 60 will save fuel, it might cost time, and that can be money. One driver reported driving from Texas to Disney World and got about 8 MPG towing his fifth wheel at 65 - 68 MPH. On the return trip, he made a point of driving 55 - 58 MPH and got much better mileage. He figured it all out and printed the results, that he saved $100 or more on the return trip, and it only took about 4 hours longer. So he saved $25 an hour by going slower!
I took a hint from this and while moving all my stuff from Southern California up to Oregon, I set my CC for 55 MPH and also got the best MPG in my motorhome. Still while on vacation, I set the CC for about 65. It does save time, and sometimes that is more important.
Also are you using overdrive while towing? Usually you should be using overdrive. Yet if you find the transmission is hunting between drive and overdrive, it is time to lock out overdrive. My 1997 Ford motorhome used to search between overdrive and drive before installing a Banks Trans Command. Now it spends more time in overdrive before downshifting.
Not using overdrive can also take away a lot of MPG.
Pushing away 65+ square feet of wind at 65+ MPH can also use a LOT of horsepower.
Good luck on your long trip!
Fred.