There are lots of great parks near Santa Cruz, most of which you can stay for at least a few days before they want you to move on. The first that comes to mind is Seacliff, great campground, beautiful beach, just north of Rio del Mar, just south of Santa Cruz. New Brighton would likely be a good bet as well. Both those sites have hookups avail for another $10/night. Most of the state parks are easy to bounce between, as long as you can swallow the $25+.night or so that they cost these days. I believe that they limit you to a total of 30 days per year in the Santa Cruz area state parks (probably to discourage the deadhead/schoolbus crowd), but I imagine there are some creative ways around that if you put your mind to it.

Wait, did you mean that you want to be as close to SF as possible? If so, you'll want to look between Pacifica and Half Moon Bay... Santa Cruz is an hour plus drive to SF under the best of conditions, ie: NOT pulling a trailer!

Hope this helps...