We usually avoid the coast in the summer for just that reason.
If you don't need hookups, there are numerous camping places in the National and State Forest land in the coast range only a few miles from the coast that are nice. Take a look at this
website for ideas near where you want to be.
Also, many of the state parks have loops that are not in the reservation system. You just have to be there early in the morning and you may not get two spots next to each other, but there is a good chance of getting spots mid-week.
Have a great trip,
Steve