Unfortunately such places don't seem to exist in California. If there are showers, then there are hoards of campers with boom boxes, etc.
So it's either showers and noisy crowds, or pit toilets and peace and quiet for me.
I found a nice little place on the shore of Lake Superior at Grand Marais Michigan. I never thought I would like it but its a little campground on the edge of town. (Grand Marais Michigan is very small)