Most CCs will disengage when the speed drops by 5mph or so due to load. You'd have to look at your owner's manual to see if it's mentioned. There is no reason not to use CC when towing out on the open road. When in tow/haul mode, the tranny and computer work together to make the best of situations. As said before, in town or hilly country, keep it off just for your own safety. My '11 F-150 with the new 5.0 does a fine job with the CC on.
J & D in Lovely Weatherford, TX
2016 Jayco White Hawk 28DSBH TT
2014 Ram 2500 Crew Cab 4X4 6.7L Cummins