San Francisco bay area, maybe a 100 mile radius ... is PACKED. Housing is outrageously expensive so people get creative and find every which way to stay somewhere ... also RVs in town now pretty much equate to "homeless" as in many just can't afford even the cheapest rent and end up boondocking on the streets.
So plan this part of the trip in advance .. northern CA you'll be fine .. pleny of great places to stay. If you want right on the coast, you should reserve in advance but unless it's a weekend, it's unusual that everything is gone.
Between the SF bay area and LA, you have to plan a bit more than northern CA .. especially if you want to be right along the coast. Inland shouldn't be a problem with a couple days notice.
The big touristic spots like the national parks and Big Sur definitely fill up fast ... and yosemite .. well yah forget about that. The other unfortunate thing that really changes everything is the massive wild fires we tend to get after Summer. But not much you can do about that at this point other than have flexibility in your plans.
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