I always use the engine brake on downhills. The rule of thumb I was told at Camp Freightliner is whatever gear you needed to climb the hill is the gear you need to use on the decline. While I admit I don't always follow that rule, on really steep inclines I am much better at watching the lower gear.

Using the engine brake will not always keep you slow enough, but he helps enough that you don't need to ride the brakes to keep the coach under control, just use them enough to take the overspeed down.