As an old time truck driver from way back, from Colorado, with over a million and a half miles in big trucks, what he told you was not wrong, however, when you do it this way, the amount of pressure you put on the brakes is critical. With semi's and air brakes, you have an applied pressure air gauge that tells you how much pressure you are braking with. In the old days, before most of the trucks had jake brakes(engine brakes), you could come down a long grade a little bit faster by applying about 5 or 6 psi to your trailer brakes going down the hill. Not enough to overheat your brakes, but enough that you could go down the grade at maybe 20 mph instead of 15 mph. We are talking much slower speeds than what people drive now, and as I said, minimal pressure on the brakes. It's not the way I drive my pickup and trailer in the mountains now, and it's not what I would recommend. For one thing, our pickup brakes these days are disc brakes, and while they don't fade like the drum brakes do when hot, the rotors will warp and crack. The main thing in the mountains is to slow down at the top of the hill, before you start down- it takes a lot less brakes to do that than to try to slow down when you're gaining speed headed down the steep hill.

Good luck!