Personally I tow, on Interstates that allow for it, at 67-71. It puts me in a rpm band that is better for the truck than 62-65 and even thou the hp requirement is higher the truck is happier and pulls better.
Goodyear Marathon's, what I have and have had on both my TT's, now have a speed rating of 75 mph when pressure is inflated 10psi over the required psi for the weight. Goodyear allows sidewall pressures to be exceed by 10 psi, i.e. 75psi max on a LRD tire. Based on Goodyear's weight charts I only need 45 psi in my tires, so I run them at 65 on Interstate trips and don't worry about 67-71 mph. I could go up to 75 psi cold, and might on LONG trips, but generally have no need for it. (This is a recent bulletin but I have towed at those speeds since owning TT's (2008). It's not often, but it does occasionally happen. I am always well under the max rating per tire and at sidewall pressures, so I guess that explains my lack of issues.)
I have seen the comment about towing speed limits being lower and I am not convinced that isn't a carryover. I also don't believe a lot of what is on the websites. Case in point is Idaho, the "towing speed limit" is 65 according to all the websites and they are all wrong. The towing speed limit applies ONLY if you are 5 or more axle and over 26K lbs, if you don't meet those then the speed limit is the same as it is for passenger cars. There is also a current push, which is about to pass from what I gather, that will raise the "towing speed limit" up to 75 to match the passenger car limit.
A large delta in vehicle speeds, towing or not, is more dangerous than going fast with a trailer. If your trailer is not stable at 65, even with a good hitch, then something is wrong... it could be tongue weight, trailer alignment, WDH setup, or even not enough tow vehicle.
The biggest issue with speeds above 65 is the tires, I think the ST rating needs to be increased to 75 on all ST tires.
