Our first year of RVing we let TomTom take us down a fairly steep (13 percent) downgrade in the Catskills. That's when we learned to pick fastest route instead of shortest route, and to not ignore signs that say "4 Ton Road Ahead". I also got the mountain guide.
I didn't know it at the time, but only the front brakes were working. I stopped several times to let things cool off, and went down the road in 1st gear. Google maps with terrain turned on is your friend. I also discovered that you can get elevation of any route in Google Earth.
