I asked the same question on the DL board about camping near Disney. We'll spend a couple days at DL and then onto SD.
I'll second the KOA.
I've camped at KOA in SD for the past 6 summers. Love it. Wouldn't go anywhere else.
It's quiet, safe and clean. Locked gates, you got a code to get in after close.
Yes it seems very pricey but its still much cheaper than a motel. I find it very easy to get to, its just a few blocks from highway. About 20-25 minutes to Sea World. 45 to
Legoland and WAP. Theres a discount grocery and a Toys R Us a few blocks away. Also a mall a few minutes east, all the junk fast foods places are nearby. We usually stop at McD's for breakfast to go, its right before freeway entrance, then before you know we're at Sea World.
We have seasons pass to SW and spend 4-5 days there each summer, as well as Legoland 1-2 days.
There's also a state park we've stayed at, can't remember the name. I think it was just a bit north of Carlsbad. I really enjoyed it tho you have to reserve ahead, we lucked out and got a site last minute, in the morning they hold a draw for those that want to stay again (without reservations).