[Mr. Adventure]

You guys are spot on about this. Everything about towing is substantially better at 55 than at 65 no matter what you're driving (and hugely better than at 70). 55 vs 65 gives you a 39% reduction in wind resistance (where most of your fuel goes), and 55 vs 65 provides a 40% reduction in stopping distance, for a couple examples. It substantially reduces the forces that the bumps hammer onto tires and hitch components. For vehicle durability, safety, and fuel economy, 65 instead of 55 is like turning your 4000# trailer into a 5600# trailer (and vice versa).

The faster you go the less stable on the road your rig becomes, requiring more driving care and attention (probably by these same kinds of percentages). In my motorhome days I found that I could drive more miles in a day at 60 than I could at 65 simply because keeping it in the lane was less stressful.