A number of years ago I talked to trans expert Manny Travao about this issue. He spoke to the parts in the trans that transmission braking can be hard on. (can't remember what they are anymore) He advised me that brakes are cheaper and easier than transmissions to replace. So what I've done since is, on long downhill interstate grades, I start at the top at 55mph and allow the coach to get to 65mph. I then brake to 55 and let it coast to 65. That intermitant braking allows the brakes to cool between braking episodes while keeping speed reasonable. On long steep 2 lane grades, depending on the grade and how winding the road is where I have to slow to low speeds, I will then use the trans to assist braking. JWID