We did a 9 week trip out west this fall and made reservations for almost all of the nights. I would not do that again. We did find that weekends can be tough at the more popular state campgrounds.
For San Francisco we stayed at this campground:
Fair Park - Pleasanton, CAIt is in a nice town and you can take the BART system into the city to visit. There are not too many places to camp in the area.
Yosemite is tough. We stayed near Mariposa outside the valley but in Wawona campground. The sites in the park are all close together and the park is very crowded.
Have a great trip!
