DH does on level stretches. It helps keep us running smooth. He has a tendency to get tired legs & while stretching pushes gas, eases off, pushes gas & eases off. Come to think of it, gives Me a smoother ride, too.
Assuming you have an automatic transmission, as long as the cruise is holding the speed and the transmission is not "hunting" or shifting up and down, let it run in cruise. A transmission temperature guage would also be a good investment since you are towing towards the max for your truck.