During my last trip (2500 miles to northern California) my truck overheated a couple of times on long grades (first on the Grapevine). I asked fellow campers and searched the internet but could not find an app that would tell me of the % grade. Then I discovered www.flattestroute.com
and it is perfect for my needs. It easily allows me to examine routes BEFORE I take them. I now know I have no problems less than 4% and have serious overheating at 8%.