Dag Sander,
How nice that you are planning a road-trip along the coast! A roof tent with a 4X4 sounds like a fantastic setup.
I'm not that familiar with off road driving and gravel roads, but I found a few off road driving trails on
the All Trails app. These don't seem to be official and are user referenced, so make sure to double check beforehand.
Were there specific areas that. you were looking to explore and go offbeat along the west coast? You may find this
campervan travel page relevant for your trip.
Groet,
Antoine