I travel a lot in places where the traffic moving over the speed limit (which might be 75 or 80 mph out here on the plains) is a whole lot faster than my gas RV comfortably manages, so on these high speed highways I try to keep up with the heavy trucks in the right lane, and if they want to go past me, they can take their chances on the passing lane where someone might be coming along at 100 MPH.
Actual speeds, if I have no wind or a decent tail wind, my cruise speed might be as high as 70-75 mph not towing, 60-65 mph towing. Into a 15-20 mph headwind, or bucking strong crosswinds, I might be down to 55-65 mph on rural highways.
For me, it is about adjusting to conditions. If it was about "best MPG" I would have to cruise at 35-45 MPH which would not be safe on highways, not legal on most limited access highways, and not very useful for making progress on a road trip.
You have to find your own MPH niche, and it will vary with conditions.
Tom Test
Itasca Spirit 29B