We use cruise control whenever we can while towing unless going up fairly steep hills or on downhills - regardless of the grade. Typically I like to let the truck 'roll' or coast down the smaller hills and cruise will often downshift to slow things down unnecessarily IMO most times...especially when the downhill is short and leads right back to an uphill stretch.
On the steeper/longer downgrades, I like to control the descent using the tow/haul mode (always active when we're towing) and the engine brake feature.
We averaged close to 12 mpg towing on our trip to Canada last summer and the majority of the trip was with the cruise control engaged.
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2014 Wildcat 312QBX
2014 Ford F350 SD 4X4 CC LB 6.7L PSD
Bob & Debbie, USAF Retired