We live near downtown San Diego and have thought about moving into our airstream. Right now it is stored in an RV storage facility and we work on it when we can. So the plan is second priority to getting it livable.
I have done lots of Google searches of trailer and RV parks in San Diego.
I've found some in National City, Chula Vista, and the Moreno District of SD. Those are closest to the beaches. Santa Fe on th 5 has a 10 year old limit. There are some in El Cajon and Lakeside. I think these are all like trailer parks that allow RVs. Not sure what you are looking for, but I hope this helps.
Keep us informed on what you come up with and what you decide to do. BTW, now is not a good time to sell real estate in San Diego. Lots of cheap foreclosures on the market.