I grew up in the Bay Area, still live nearby and I'm still very familiar with the area. I agree with others that you should focus on the East Bay, if you can. Marin County is just too expensive for just about everything, same with SF. Look for RV parks within biking distance of a BART station or ferry terminal. Another option might be Vallejo, which has great high-speed ferry service direct to SF.
You should also check out
this fantastic blogger, who lives in an RV in the Bay Area and writes about living frugally and early retirement. In fact, everyone should check out this blog.
