Even with the same set-up as I have I know everyone will have a different experience and opinion of what is right for them - this is just my opinions and reflections on what I learned with my set-up. Life is a learning experience - will I continue to experiment? I doubt it. However, I will continue to watch what is happening with my fuel economy.
I have been pulling trailers for over forty year and I always knew that I would get 9 - 10mpg.
My main purpose was to dial in my "sweet spot". With over 20 fuel stops and many different conditions I was able to find that "sweet spot"! I was looking for fuel efficiency and driver comfort. Sure I was comfortable going at a higher speeds but at what expense?
Would the truck hold the speed with the cruise control active without massive throttle response to the conditions? At 65mph@2100rpm that was possible. If I tried to run in 6th gear the transmission tended shift down to 5th often and the economy suffered.
Happy camping & Safe travels!!