If you are thinking about going up the center and coastal regions of California; Once you get out of southern California, there are a few interesting fire and forest roads, campgrounds north of Ventura, in the Los Padres National Forest, that exit terra firma from Hwy 33. Pismo Beach offers a unique place to camp and use that 4x4 lever and skills with little chance of getting stranded (Lots of potential help to pull you out of a rut, if you happen to get your camper stuck). Morro Bay, Cayucos, and Cambria offer nice coastal views and camping spots. Bypass the Bay Area, not much offroading or camping opportunities there
There are quite a few interesting forest service roads through northern California, that might be worth a couple days of exploring. We've gotten off-road starting at the north side of Indian Reservoir and unimproved camping at Hough Springs, which is north of Clear Lake. We've stayed mostly on dirt around Lake Pillsbury, over Hull Mountain, and up through Mina, the tiny town of Kettenpom. The trinity and mendicino forests lie between Interstate 5 and 101. Nothing too tough, fireroads and such definitely off the beaten path.