http://www.springsatborrego.com/ $37/night?
http://www.palmcanyonresort.com/rvpark.htm $32-36
Overview of sorts
http://www.socalcamping.com/000Anza/main.html
My thought would be late Feb, early March for a chance at flowers. It's not too hot then either. Usually quite mild.
So this is a possible date and destination... I like the beach too.