Guajame is small but nice and it's a San Diego County (not state) park. There are a number of others run by the County, they will be much more available than the State Parks and also cheaper.
Google "San Diego County Paks Campground Reservations"
If you are coming to San Diego you might also want to consider Lake Jennings. It's run by the Helix water District, has full and partial hookup sites and can be reserved online. One caution on this one - many of the partial hookup sites may be a challenge when you need to level! Also most of the partials are on a road that prohibits vehicles longer than 25'. Great for small class C's, tent campers and small TT'S but not suitable for the larger ones.