I am pretty much like everyone else out there.
I try to run at 65 and will switch in & out of Cruise Control based on the traffic flow and terrain!
A good rule of thumb is that for every 10 mph increase in speed you will see a 10% decrease in fuel economy.
I have only towed with my current set up around 4000 miles. So believe it or not I am still working on finding the sweet spot. I suspect that it is right around 63 - 66 mph! When you don't run the same route all the time it can be a bit tougher to dial it all in.
For those of you that tow at 80 - what the hell is your hurry? If you are in that much of a hurry take a plane! !
Happy Camping & Safe travels!