There are lots of apps that can tell you where the rest stops are, but they don’t know where you’re going. It can be a pain to check the rest stop app, then go back and check your GPS, then go back to the rest stop app, etc.
Since Google Maps, on my iPhone, is doing my navigating, it knows the route I’m going to follow. I can ask Google Maps to search for Rest Areas along that route, and it can add one as a stop, giving me the directions. Everything you need, all in one app. Watch this video to see exactly how to do it.