In April - 17th thru 23rd - we went up that route to the Thousand Trails campground. There was plenty of warning/ signage to not take a 'large' vehicle up Old Priest Grade Road, but to use the longer, easier grade, better road.
One other thing I'd recommend about using a GPS - TURN OFF THE VOICE DIRECTIONS.
They give a false sense of urgency about making a turn if the view out the windshield makes the turn questionable.
A GPS (or Google Maps or any other app) is an ADVISORY tool.
It is always the driver's responsibility to research the day's drive ahead of time, look for potential problem spots and plan the best way past them.
