Local trips the phone GPS is fine. But for longer multi state trips towing the RV I prefer the stand alone GPS. I have a Garmin 760 RV LMT it is designed for RV use. It's not perfect but when towing the RV it does a better all around job than my phone.
The stand alone keeps me on main roads. Google maps will take me down the smallest street is can find. Google map will also take a short cut down a small rd. The Garmin keeps me on the bigger roads.
I don't mind Google Maps on my phone for daily or local use , but while towing the stand alone keeps me on the better path.
Never trust any GPS 100% always verify via paper map or 2nd source.