Doheny State Beach has an RV park right on the sand, admittedly right under the flight path for LAX. The area around Disneyland and Knotts Berry Farm has a number of commercial parks. Up in Valencia, there is a large RV park convenient to Magic Mountain Amusement Park.
Southern California is huge area over 200 miles north to south, San Diego thru Los Angeles to Santa Barbara. There are lake campground but they seem to be in the north end of things near Santa Barbara. LA is ocean, amusement parks and and tourist sights. There are CGs up at Big Bear Lake at 8000 feet in the mountains. San Diego is a real treasure and they have a number of decent parks: Mission Bay, Chula Vista, the SD KOA, and a few others.
What are you looking for in cities?