All "young speedster" scolding aside, there are times when running up to 70-75 is needed and I've wondered the same thing. I've done it for short periods and everything feels just fine with my setup. Keeping in mind my setup is a newer 3/4 ton truck with a newer TT (both less than 3 years old). Good tires on all, PSI checked recently, bearings on the TT packed, etc, etc. In general I feel best towing at 60-65 on the interstate.
The "short periods" I describe are usually things like getting around a pack of 18-wheelers and not wanting to hog the fast lane. Honestly, I've never taken my TT on a long interstate trip. I'd have to imagine moving along at a consistent 70mph wouldn't be out of the question. I think a lot depends on your equipment (tires, tires... did I mention tires? :)) and the terrain.