With my '97 gasser with 4.10 gears I would stay at about 60mph because the truck was old and only got 7-8 mpg when towing. Engine turning at about 2,900 rpm.
Now that I have a 2500 CTD I still only tow at 65mph because I can keep it in 5th gear overdrive all day long and still get 14mpg with the same trailer. Only turning about 1900 rpm.
I could go MUCH faster, but why?
As someone else mentioned being able to stop and keep control of the truck and trailer in the event of a blow-out or some other situation is more important than getting somewhere faster and putting your life or your family or others on the road at risk. Saving a few minutes or an hour on longer trips is just not worth the risk.