If you get there early and maybe on a Thursday, you'll have a pretty good chance. Sites fill up pretty quickly, especially if it is hot in the Valley.
6/2020 We arrived around 4:45pm on a Thursday and got the very last FCFS site. (The sign at the Hwy said Campground Full, so maybe that deterred others from coming in and double checking with the host like we did.) Friday morning by 10am there were 2-3 vehicles who'd arrived and were waiting for the next available spot. If this experience is typical, it looks like you should arrive pretty early on a weekend, and have greater wiggle room during the week, but still somewhat risky.
Most of the sites are walk-ups. Don't expect a beach site on the weekend unless you are there before Friday. No showers though. But you can pay $2 for a shower just up the road at Beachside SP.
It fills up pretty quick. We always make reservations 6 months in advance. But when we were there this family came in and got Camp spot. They do have them first come first serve. just got to be lucky good luck.😊
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