60-64 on the interstate. I prefer to travel the side roads when possible. I can go slower and there is more to see.
I think many RVers travel at or below the speed limit, that seems to give the best MPG.
You may also hear about the Type A, high strung people who brag about driving/towing at 80+ mph. They are more concerned about getting from point A to point B and don't enjoy the scenery.
Drive at a speed that you are comfortable with, just don't hold up traffic by going too slow. Pull off and let traffic get around you if traveling on two lane roads.