Technically, peak season in California is summer. Everyone is out of school heading for the beaches. But, the best times to visit coastal California (north to south) are September, October, and sometimes even into November.
Not only have most of the crowds gone away, but the marine layer (aka fog) and the associated winds have gone away too. Leaving far less crowded, beautifully sunny beaches up and down the coast.
By late October, early November the far northern coast will usually see winter rains beginning to return. So start North and head South. I believe this also applies to the Oregon coast as well, which has far more scenic drives right along the coast that are RV friendly, and heading North to South allows larger vehicles easier access to all the scenic turnouts.