Camp Pendleton lets you camp right on the beach. Also, if you are okay with the beach not being on the ocean there is Magdalena Ecke, which is in San Diego, right off the 5 (great kayaking there). But most places don't let you park on the sand. If you are willing to walk 100 ft from your RTT to the ocean, then there is also Doheny State Beach by Dana Point, Thornhill Broome Beach Point Mugu State Park if you don't mind the wind, Emma Woods State Beach is up in Ventura, Jalama Beach in Santa Baraba, up there there is also Gaviota which my dad loves. In San Diego, there is San Elijo, and South Carlsbad (but there you are up on a cliff).
P.S. - RTT and San Diego