Not in So Cal any longer. The year round camping is great though.
Some of our favorite spots in so cal.
Spring, Summer, and fall we Liked to hit Mt Pinos specifically McGill Campground. 1 hour north of Santa Clarita up Interstate 5 (The grapevine) It's first come first served but we never had a problem finding a spot even on holiday weekends. (I do recommend heading up on Thursday for a holiday weekend though.) It does close for the winter as Mt Pinos does get snow. Like most forest service campgrounds in So Cal; McGill is a dry campground so you have to bring your own water. Elevation is approximately 7000 ft so it's a nice escape from the heat.
Check out all of the options in Los Padres National Forest. From Mt Pinos to Big Sur it is one of the largest national forests in the US.
In Winter we would hit the beach campgrounds along US 101 especially El Capitan. These are also first come first served in the off season but there is usually plenty off availability in the off season.
If you try the beach during the camping season you will need a reservation so plan ahead and book early.
These were among our favorite spots in So Cal.. Now we are in Colorado. (The camping here is terrible
so don't come here for camping
)