I have a 2009 D/A and the grade brake does not work in the manual mode. You can slow the truck down some in manual but not like you can with the grade brake engaged. This is also noted in your owners manual.
You also have grade braking on cruse control. I pull a 33' trailer and when I start down a grade I set the cruse control at the speed I feel is save and the transmission will control the speed. Just don't jump out of the seat when it downshifts, especially if your cooling fan is also running.
Engine brakes are great but they are also expensive as an add on. I suggest you try it stock before you spend that much money and void your warranty.
Try it nyou will like it.
Doug
