I have a 2013 F350 dually diesel. I full time and have driven up and down many of the 'hills' in Wyoming, Colorado, California, etc. I sure do not claim to be an expert.
I use Cruise all the time but never going down a steep hill - or any hill where I think I may exceed my cruising speed by more then a few MPH. I actually do not see how that's possible because Cruise kicks off the instant you touch the brakes.
I usually adhere to the rule that says you go down in the same gear you came up in. Depending on the grade, I may go over the top in 4/5 or in 3/4. The trans will shift down (as intended) if you press the brake pedal, unless the computer decides that shifting down would result in excessive RPMs. I have seen the RPMs touch 4000 several times. That is when I will get on the brakes until the rig is slowed down a little. Have never seen any warnings.
I agree with the others that you just cannot blindly stay in cruise on the steep downhills. You want to go over the top slow in a low gear to give yourself some margin for the speed to build. Know how long the downhill is and if there are curves. Pick your starting gear and know what's going to happen. Use your brakes sparingly but use them when needed.
I may have to experiment with the transmission's 'manual' mode as this may give me better control.
* I turn on Tow/Haul before I hitch up in the morning and don't turn it off until I stop for the night.
