Exhaust brake all the way. Just got home 3 weeks ago from a 8k mile trip. 4.5 weeks. PA to Cali and back. Plus a lot of stops in the rockies. Going down the big hills for 11 miles with 6 and 7% or more I never touched the service brakes. Exhaust brake did it all. To be honest it often times slowed me to much and I shut it off. Went another 1/2 mile and then turned it back on as speed built up again.
Once you have done the rockies you will always think very carefully about GVW, loads and brakes.
Having been over them 22 different times I have never had a problem. But I don't push anything to its limit. Things last longer when you don't. I need to update. We sold the Zephyr and got a 33ft fiver. LOVE IT. Pulled it with the Dodge.