It's difficult to recommend camping for those coming to this area. Anybody coming half way cross the country almost surely wants to be right in San Diego and most likely on the water. Study a map of the city closely and you will see that we have some wonderful bays just inland off the ocean. They provide the best campgrounds. So some really nice expensive campgrounds typically end up as the default recommendations.
Directly on the ocean - if that appeals to you - there is also some camping but more difficult to get into. Out in the mountains east of the city about 50 miles are a couple of national forest campgrounds ($20/night). But you'd be driving maybe a hundred miles a day "commuting" into the city sites. That would pay for an expensive campground right on the water.
Off the top of my head, check out Santee Lakes. A park like setting that will not disappoint. Base yourself there for a couple days and check out some of the places on the Bays.
As an aside. Coming across Arizona and California this time of year will be hot. We have often seen over 110 coming and going. You will climb up into mountains starting about 80 miles east of the city and then drop down out of them as you near the city.
Also. That famous San Diego weather only exists right near the ocean. Go inland across a low range of hills 10 miles to the east and you may experience much warmer temperatures. Not desert temps, though, by any means.
And one last thing to note. We are famous for June Gloom during which time the beach area will be covered with a marine layer of low clouds. Combined with a steady onshore breeze off a cold ocean current it can be somewhat cold in the mornings or until that layer burns off. It can last into July.
If you see a Winny Vectra heading your way across the freeway give a wave. It just might be us doing our yearly run over to the Lake Conroe area.