Towing speed?

I tow across the USA annually. I've done this for 20 years. I use the interstate to get from state to state. I normally drive at 67 mph. I am retired but I also have other things to see and do but drive. I also break the rule and drive what ever it takes to get from RV park to Rv park, this can vary from 5 hours to 12 hours on the road.
We currently are resting in a park and tomorrow we go for a 9 hour ride. This is our style as we know these roads well and our friends await us.