MFA was spot on.
I only use cruise control strategically. As long as the TC stays locked then cruise is fine and does not hurt MPG. When the transmission tries to "help" maintain speed, all bets are off.
Learning my shift points was key. In D my min lockup speed is 40 mph on accel and 35 on decel. In OD it's 45 accel and 40 decel. OD is my truck's default mode, and my power to load ratio is high enough that most of the time I can tow in OD. When inclines are too much for OD then I adapt first by gaming the hills like a trucker, and manually select D mode if needed. On a typical 1100 mile trip between MA and IL there are only a few miles of grade in PA where D is needed for my rig (power/load). In a rig with more weight and drag I might spend more miles in D than OD.
Goal #1 for me is minimizing transmission temp. That means not letting the TC unlock. Stirring ATF generates heat. Heat kills trannies. Meanwhile, torque converters hurt MPG.
On long down grades I used to think coasting was best for MPG. One thing my ScanGauge taught me, however, was that engine braking is often better. Instead of idling at 0.5 gph my injectors get shut off so zero fuel gets burned. My trans fluid is usually warmer going down than up as a result. Doesn't seem intuitive, and may not work well for others but works for me.
Another trick I learned was that flashers are my friend. They ward off tailgaters like deet on skeeters!
Cheers
KB