ST Speed Rating
I've done a little research on the speed rating of the ST type of tire typically installed on travel trailers. It looks to me like the speed rating is 65 mph. That strongly suggests to me that 65 mph should be your upper limit when towing. Especially if it is 95 degrees and you are having a 700 mile towing day. I would think that one would want to be at or very near the maximum pressure rating for your tire. That, of course, varies depending on whether you have a B, C, D, or E load rated tire. It seems that the ST tire sidewall construction and higher pressures are all about minimizing sidewall flex and sway.