First of all, I have to warn you that you'll get a friendly bum-kicking here for calling the city "San Fran". The locals allow the city to be called either "San Francisco" or "The City". Abbreviating "SF" in writing is also okay. :)
Now that we have that out of the way, if you are truly looking for back-country camping then I recommend Henry Coe state park. It is south of the bay area in the Morgan Hill area and is the biggest state park there is. It is a fantastic treasure for backpacking so long as you don't go in the summer/fall when it is hot and dry. This winter and spring it should be glorious with rolling green hills and very few people. Have fun!