For the 22 years I have been towing a 15,000lb 38ft 5th wheel I've had luck at 60 to 62MPH (in 55MPH towing zones) never been challenged by a state trooper/ highway patrol. In 70MPH towing zones I still stay at or below 65MPH for safety more than any other reason. My c4500 is smooth at 75MPH towing but I feel safer at the slower speed after seeing some results that occurred to other 35 to 40 ft 5th wheels at speed.
The speeds you are talking about are a little scary for a couple of reasons:
> Primarily tire heat for the TH. Blowouts aren't pretty.
> And being able to stop in time and/or avoid idiots