An RV specific GPS (when RV enabled) will tend to route you on commercial grade roads where your RV requirements (length, height, weight) will be supported. The Google GPS assumes you're driving a 4WD drive car and will route you on the shortest path to your destination including dirt roads, low bridges, and tight switchbacks. I use both - plan and drive the route on my Garmin RV-660 but use Google GPS on my phone for current road conditions. (The Garmin also has a good backup camera.)