I learned this lesson by experience with a Hill too long and steep decades ago
http://www.explorenorth.com/library/roads/bella_coola_hill.html
We had a 1992 3/4 ton GM van pulling a tent trailer. I started down in 2nd gear, using the brakes on each switchback curve. My daughter had her window open and said there was a burning smell. I stopped and found there was indeed a hot smell from the wheels. A few minutes later I bravely proceeded in low gear and soon came to a rest stop where I could safely park. There was a melted car there. The aluminum engine had the shape of a puddle on the ground. After a good rest to cool down we proceeded down in low gear. A good lesson!