I don't have an exhaust brake on my truck. It would be nice.
I fought with the electric brakes on our fiver, trying to get the lock up which is discussed here. Never could. Then when I had to re wire the brakes and harness because of the inadequate wire size, I still could get no lockup even with new brake assemblies.
So I replaced them with disk brakes. Now I definitely can lock up the brakes in order to adjust the controller to the suggested setting. The rig stops very well now, and I don't really feel the need for an exhaust brake.
It is an expensive upgrade, but now I feel very much at ease when we are coming down mountains.
Our fiver is an 05 model, so is pretty old, but we think that adding the upgrades to it makes it almost as good as a new camper, and in some ways better, since we can't afford a top of the line fiver which already has all the things I have added to this one.