Lots of boondocking in the desert -> out by the Slabs, in Anza Borrego State Park (lovely place to boondock), around Yuma. But you will be hard-pressed to find anything on the coast. Most of the coast in CA is pretty tightly regulated, and very expensive. There are some Casino's that allow overnight camping along the way, but not many.
Within San Diego there is no "official" boondocking by the beach and overnight parking (in your rig) is not allowed on most streets. You can legally free-camp during the DAY at Fiesta Island and you will see people "stealth camping" in the parking lots at night around Mission Bay, but it's not officially allowed (up to you if you want to try it). I have pics of the day-camping area
HERE.
There are some Casino's that allow boondocking for a few days in San Diego, but they are not close to the beach. We've stayed at Sycuan Casino in El Cajon and they're not bad. Large lot, easy to park. You're about 30 mins from town however. I have a link to our review of Sycuan
HERE.
