I've read some of those posts, and I think it is important to understand that the jake brake effectiveness is directly proportional to the engine speed. The reason it slips to 4th gear is to assure a reasonable effectiveness to the engine braking. The danger is the engine speed may go over the recommended limit if you're already going too fast. I've looked at this again and again, and everything I've read says leave it the way it is (4th gear), and change driving habits.
For example, those road signs give you notice of those steep declines and the literature I've read says crest the hill at the speed you want to descend it at. I don't think many of us are cresting hills over 60mph which is probably what it would take to overspeed your engine if you then activated the jake brake. Rather you're probably cresting the hill at a speed well within the engine's capability to slip into 4th, generate significant engine braking (due to the engine speed), and you safely go down the hill.
When I first started driving my coach, I waited too long, didn't look ahead, then was going to fast, and pumped my brakes to cut down my speed to under 60, then used the jake.
That's not the way you should do it. At least based on what I've read. So, you could manually downshift first to 5th gear for example to slow down a bit, use your brakes, then set the jake.
The problem with leaving it in 6th gear is you won't have an effective jake due to the lower engine speed. I think the engineers who designed this system have it right.
For me, when I crest a hill I make my decision, and if I am wrong initially selecting the jake, I take it off temporarily, but before going too fast I set it. If I need more, I select the 2nd level jake. In other words, going downhill I rarely use brakes any more. I control my speed using the jake.