I'm not aware of any RV parks in SF. Why don't you just move to Oakland/Berkeley? You can get downtown in about the same amount of time, if not less, and you have a better chance of finding a place with a backyard larger than a postage stamp.
Check out this guy. He lived in an apartment in the SF area:
<a href="http://earlyretirementextreme.com/rv-vs-apartment.html" rel="nofollow">http://earlyretirementextreme.com/rv-vs-apartment.html</a>