If terrain is rolling hills or involves long stretches of even gradual uphill you may find yourself downshifting to maintain power. If you have to shift more than you want to shift stay in a lower gear. Adjust speed.
Going down hill shift up to improve fuel economy. That is if you don't need the engine and lower gear to prevent picking up too much speed (engine braking) on the down hill.
Whole point of having that many gears is to allow engine to spend more time in the "sweet spot" of it's power output.
Automatics with overdrive have an OD off feature in part because the slight push/pull of the trailer from minor changes in throttle, bumps or small brief changes in up/down grade of road can cause the transmission to keep shifting back and forth between OD and next gear down. Hard on the transmission to shift back and forth in highway gears.
Think about going up a hill on a 10 speed bike. If you drop down to a lower gear you pedal faster to keep up the speed but you can make it up the hill without much effort. Keep it in a higher gear and it is a lot of work.
As a bike rider I understand what you are saying Roy. Perhaps that is another reason why I like to stay out of the low RPM range when towing.
I just thought of something while reading this post.
A good indication of how the engine likes the gear ratio.
With the trailer connected and running down the road put the transmission in 4th gear and push down on the gas pedal for 4 or 5 seconds and watch the speedometer, noting an increase in speed.
Next try the same thing in 5th gear and note the speedometer change.
With my '05 2.7 5 speed tacoma, I never use 5th gear when towing, 4th gear is good up to about 85 mph. WAY faster than I drive anymore. Let the RPM's do the work, these engines are not made for low end torque