I would guess that Forest Service campgrounds are your best bet. But ones close to the cities in So Calif are probably busy through out the summer. The best way to avoid crowds is to go when the crowds don't - i.e. during bad weather.
BLM also operates campgrounds, but in your area they are most likely to be in the desert.
Some FS districts allow 'dispersed' camping, generally any out of the way spot. But check your local districts first. I suspect they aren't as generous with this option.
BLM also operates campgrounds, but in your area they are most likely to be in the desert.
Some FS districts allow 'dispersed' camping, generally any out of the way spot. But check your local districts first. I suspect they aren't as generous with this option.