I'm know that I'm not the first one to think of this, but when I get into rolling hills where the cruise control may force a downshift, I release the cruise control on the hill and feed just enough fuel to keep the transmission in 6th gear for as long as possible. The cruise control cannot anticipate an upgrade, so using a little extra throttle before the upgrade helps keep the transmission in 6th gear even if I don't disengage the cruise control. Over a long trip, I've increased the fuel mileage by 0.2 to 0.3 mpg. Plus, it gives me something to play with!!!
Just a thought.
W8RLM - Bob
2006 Winnebago Journey, Model 39K, 2013 Dodge Dart