The closest place I can think of from San Rafael would be
Cow Mountain OHV. If you can drive little more on N.101, you can visit
Lost Coast, Redwoods. There appears to be many camping sites along 101.
quote from Northern California Backroads & 4-wheel Drive Trails
Lost Coast, Redwoods
Location: Along the Pacific Coast between Fort Bragg and Eureka.
Difficulty: Easy but rermote. Usal Road is subject to deterioration and can be rutted and muddy so avoid when wet. It is also narrow and steep in places. Be careful in the soft sand at Usal Beach. The drive improves north of Four corners when you'll encouter some local traffic.
Features: Explore remote coastal terrain. Enjoy beautiful ocean views and camp at historic Usal Beach. Pass through Humboldt Redwoods State Park where you'll see some of the largest redwoods trees in California. Take a side trip through Sinkyone Wilderness State Park to remote Bear Harbor. Consider side trips on Saddle Mountain Road and Smith Etter Road. Both roads have outstanding high-elevation ocean views. The trip ends at popular "Avenue of the Giants," a major tourist area for viewing redwoods.
To get there: Take Hwy.101 north past the tiny town of Leggett where Hwy. 1 intersects. Turn left on Hwy.1. Go southwest 14.7 miles to Mendocino County Road 431 on the right near mile marker 90.88.