I have never owned a travel trailer, but when towing utility trailers, boats, etc I always just judged by the way the tow vehicle was shifting. On flats and slow rolling hills where downshifting was occasional and smooth, I would use cruise control. When the hills got steeper and/or longer and the downshifts got to be frequent and/or harsh, I would kick out of cruise so that I could better control my speed and try to reduce some of the excessive shifting.
I have tried it both ways and found that I consistently saw a 1 mpg improvement by NOT using cruise. Cruise would try to maintain constant speed and without cruise I would take a run at a hill (judgement call) and then not push down on the throttle as hard. It is something you need to try and decide for yourself if it will be beneficial or not.