Flatlanders. At some point if the "hill" is steep enough and the load heavy enough, the engine will not hold it back. You gotta use the brakes. Otherwise the diesel will scream and cross the redline.
I have driven loads of coal and oilfield junk. I don't care if the engine braking is by Jacobs, a retarder or the variable vanes in my 2012 Ford, you need to help it some with the brakes.
Besides, where do you find a steep mountain that has a continuous 45 mph? I normally see 50, 45, 35, 25, and even 15 mph turns. How do you not use brakes and cut out the cruise control?
