I've driven various models of trucks from all three brands and towed with them. I like a cruise control to hold the exact speed it's set for. If I set it for 70 I don't want to be going 69 nor 71. The cruise in my Chevy's has done that quite well. The cruise in my Dodge was set up more like what you're describing. It would allow the speed to drop 5 to 7 mph below the set point going up a hill before it would downshift and speed back up. Not at all my preference. However, I've heard other people complain about the cruise making the truck downshift too much and saying they'd be happier if it would just stay in top gear and slow down a bit. Just a different preference I guess.
I think dropping a gear when towing in the hills is probably you're best move.
One another note, how long has it been since you changed the fuel filter?
