The newer trucks are pretty smart--you should have a Tow/Haul feature for the transmission that will assist the shift points while towing. Believe it is a push button at the end of the shift lever (Fords, no sure on GMs). When you use it, a light should illuminate in the dash area to indicate it is active. That is the first line of offense when towing in hilly areas. You can always manually shift the trans when you find the speed falling on uphills, and if you gain speed on downhills. But the T/H feature should do this for you.
Depending on the horsepower/torque of the 6.0, you may find it necessary to lock out overdrive (5th gear).
Spent Nov-Feb between Wooster and Tiffin working on the pipeline--I feel for you guys who live there in the winters...
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired