You need to check the Reservecalifornia.com site. The RV sites with water and electric are difficult to reserve. I think, from your profile, you are too long to book a "tent" site. Those are 25 ft. or less with a side x side spot for your truck.

New reservations are available at 8am pst 6 months in advance. I have noticed that people will cancel days before so be on the lookout for those at 8 am.