Allen.....On yours, once you turn the switch to "LOW" or "HIGH", the engine brake system is active, but not engaged. If you are driving down the road and take your foot off of the throttle, the engine brake will engage. It will stay engaged until you step on the throttle or drop down under 15mph (approximately).
Your engine brake will also engage while using cruise control. If you're descending a grade, the engine brake will allow you to gain about 6 mph above where the cruise speed was set and then engage. For me, this often has me going to fast. When I start down a grade, I decide if the engine brake should be on "LOW" or "HIGH". This comes from experience. I also thumb down my cruise speed, 5 times, so that my speed stays where it was set. Once the brake releases, I thumb the speed back up.
Every time your engine brake engages, here is a light on the dash that illuminates. You can also experiment a little with your engine brake. A few people recommended shifting to 5th or even 4th (depending on the grade) and then setting the cruise at a slower speed to maintain that speed on a steep grade.
Two weeks ago, I tried the downshifting technique on some really steep grades in Oregon. I shifted into 5th (engine brake on HIGH) and then allowed the speed to drop to 50 mph and then set the cruise control. The coach maintained the 50 mph all the way down the grade. If it started to increase a bit, I just thumbed the cruise speed down one or two times. This worked well and made for a comfortable drive.
Don & Mary
2014 Newmar Dutch Star - All Electric - 450 ISL
2016 GMC Canyon 4WD Crew Cab