I know these things aren't exactly fuel efficient or bought for that purpose but what sort of mileage are you guys getting with the camper on and is your rig gas or diesel?
When I usually set CC at 60 mph, when I am bored on flatland, I "catch" big semitrailer and drive in its tailwind. That sometimes is reaching 80 mph, but usually I go 70-75 mph and the drag covers for speed mpg penalty. Meaning I can follow the semi at 75 mph and still burn the same fuel I do, while driving 60 mph alone. Turbulence is a bit uncomfortable, but that keeps me from sleeping on boring drive.