I run 75-80 as a matter of routine. The only thing that slows me down is a lower speed limit, or a rare hill that is too steep to allow the truck to maintain speed.
BUT.... I have a stout dedicated TV just for towing, and tires/axles that have been upgraded a lot over how the TT came stock.
I do not believe in running any equipment close to it's max rating.... And that means that most ST tires run at 60 MPH are very close to their max rating! Even as some of you have admitted to briefly exceeding 65 on a downhill with STs.. That simply is not safe. Any tire should NEVER be run in excess of it's rating. Just as a motor with a 6000 RPM redline should NEVER exceed that redline. Every time a limit is exceeded, the risk of a failure increases dramactically.
IOW, most TT/TV combos on the road are too close to several ratings to be "safe" at 60-65 MPH. 50 MPH should be their max to leave them a reasonable safety margin.
JMHO.