I was just on a long trip with my first encounter with lots of steep hills.
My exhaust brake (made by the same company that makes what some call a true Jake brake) will engage when on cruise. I would watch the speed build up going down hill and just right at the limit of my own comfort level the exhaust brake would cut in and slow things down. On steeper hills I would still use the service brake quickly to slow down a bit more. Then if I did not engage the cruise the exhaust brake would would turn on whenever I took my foot of the accelerator.
Based on previous discussions on this forum, not all exhaust brakes work the same way - some fight with the cruise control (my 99 DSDP did) and some have no issues. Some are called exhaust brakes, some engine brakes and some are Jake brakes but they are all generally meant to do the same thing - slow you down within the parameters specified by the manufactures.
Chris