As others have said, your main problem will be finding a spot you can stay in for a month. I believe maximum is 14 days, and during summer months unless you are sitting on your computer at 8 am logged in and ready to make a reservation, 6 months in advance of when you want to go, forget getting a site in any of the state beach campgrounds.
But, assuming those are not issues, in So. CA for on the beach camping, best state beach campgrounds are Bolsa Chica and Doheney. Bolsa Chica is basically a parking lot, but right on the beach with only a narrow bike path separating you from the sand. It has water and electricity hook up, with a dump station in very close by. Doheney, you are practically on the sand, but there are no hookups.
Other state beach campgrounds where you can get an excellent view of the ocean, depending on the site, are Carpenteria, El Capitan, Refugio, San Clemente, Crystal Cove, South Carlsbad, and San Elijo to name a few. I have stayed at all of these, and they are all nice campgrounds. Some have no hookups, some have partial and some have full hookups. Obviously, the size of your RV will make a difference as well on whether you can find a spot or not. These are not on the sand, but mostly on bluffs overlooking the ocean.
For reservations, try ReserveAmerica. If you look for spots in San Diego, Newport Beach and Santa Barbara all of these campgrounds will appear on the list.
For general info, park brochures and campground maps, go to https://www.parks.ca.gov/ParkIndex
and look for any of these parks/campgrounds by name.