Going back and checking Caltrans,the section from 101 junction to Legget is closed to Motorhomes over 40',except for a couple of sections. If I remember correctly,there is a sign past Marin City going north on #1,at the intersection of the Panoramic highway,that says buses and vehicles over 30' prohibited beyond this point.
The last time I drove that section northbound,I was in my suburban,it was VERY foggy and I had some crazy woman in a BIG BMW sedan pushing me hard. She passed me on a blind curve and 1/2 mile later pulled off in a coffee stop.