For a small trailer like that, you can probably use cruise control most of the time. The big reason that they don’t want you to use cruise control when you’re towing, is the extra weight tends to make the transmission hunt between gears. Also, you don’t have a sense of how hard the truck is working to tow your load (ie you don’t know how far the cruise is depressing the throttle. It might be holding the pedal to the floor to maintain speed whereas you would know that if you were using your foot, and would back off.)
Turn it off when you get to hills or if you see that the truck is downshifting to maintain speed.