I'm kind of in the same boat. A few times last summer I just showed up in the evening and got lucky. The Ventura area is a good bet because there are a few state and county campground to try. Emma Woods is one we have had good luck with. A few weeks ago we really wanted to go somewhere so we went to a private campground in San Dimas. We got a spot with an awesome view and the kids liked it. It was only 20 minutes from home, but it got us out. Every year I tell myself that I'm going to book 7 mos in advance and every year I don't. Maybe this will be the year.