Del Mar Fairgrounds

In most other locales it would be mediocre at best, but since its San Diego and $40 it's great!
Long story short, it's a very secure place to leave your rig and go elsewhere all day, but don't except much more. More of an industrial complex than typical fairgrounds or RV park. Gated with security guards and lots of cameras.
*Important, they close down sometimes (~week) for shows, so you must call Juan (619-240-0736) to make sure its open for your specific dates. No reservations, and you literally have to call him to let in you when you arrive at security anyways.
Full hook-ups (with sewer). Cement parking lot, all spots are parallel to each other, ~10ft wide, all back-ins to a chain link fence, generally people park one space apart from each other so they have room to open their doors. It's basically a storage lot so you never know what you'll get. Your tow vehicle has to park in a designated area across from you ~40ft away. Bathrooms and laundry facilities are not kept up, but they function.
Noise from trains is minimal but there can be trucks and work happening on both sides of your spot during normal business hours (especially the closer you park to entrance, spot #1). Large cell phone tower by spot 39. You are in a city and in a discount lot, so you'll get people accordingly. Overall it's the cheapest and safest place I've found to park my stuff and go away all day.

We spent 3 days at Del Mar Fairgrounds RV park. The fairgrounds are next to the San Dieguito River which rises and falls with the tide as it empties into the Pacific Ocean. The park was neat and clean and VERY quiet which was great! There's a dog beach not far from the fairgrounds. Everyone was friendly. Ask for Juan he's very helpful! Definitely a great place to vacation! We will definitely stay here again!

I can’t say enough about Juan who is the manager of the RV area. He is kind and courteous and always welcoming.
We are from Canada and have stayed here 3 years in a row and wouldn’t park anywhere else.
The river is right next the sites and it’s about a 20 minute walk to the beach.
If you are here for Fairground events or the Race Track then this is your spot. You are literally steps away.
Easy on and off to the Hwys and lots of great places to eat near.
We always felt Very secure and comfortable no loud parties, everyone is very respectful.
Can’t wait till next year!

Camping behind the Del Mar racetrack is probably San Diego's best hidden secret. The RV spots are located on the south side of the racetrack grounds and are less than 100' from the entrance to the lagoon. Each RV spot comes with power and connections for sewage and water. The staff is very knowledgeable about the area and incredibly friendly. Depending on what activity that the fairgrounds is hosting you'll have a front row seat for the activity. My son and I love bringing the RV down for the Fred Hall Fishing show as well as the Good Guys Car Show. I highly recommend this off the beaten path campground.

We stayed here in March while on a RV trip across America and we stopped in San Diego to visit family. The fairgrounds is just a parking lot, but it served our needs well. It was in a great location and had the amenities we needed such as FHU, laundry and showers. The wifi was great, which was very important to us. Verizon and T-mobile worked great. Yes, the train goes by all day and night, but it didn't bother us at all. It wasn't crowded, which was very nice for our stay. This is NOT a "campground" or anything of the sort if that's the experience you're looking for, but it was perfect for us at the time. I was actually thankful that the fairgrounds makes this available outside of their fair dates. I saw a security guard driving around in a golf cart and there is a camp host on site. The only negative would be that the showers were run down and dirty looking. I did see someone cleaning them, though.