Hi there. We haven't taken our van to the States, but I have found several pieces of info for you.
From 2015: https://www.reddit.com/r/vandwellers/co ... s_angeles/
This is a list of Walmart parking lots: https://www.allstays.com/c/walmart-cali ... ations.htm
BUT "...As mentioned, some cities don't allow overnight campers, Santa Monica being extremely strict. As you enter the city, there are signs advising that any vehicle over 7 feet high (or 8 feet, not sure) need special permits to park. Add to that, pretty much all of SM is permit parking only.
You can try state parks. There are several in Malibu, but it's pretty hard to get out of there via public transit, if that's what you are hoping for. http://www.parks.ca.gov/?page_id=1029
The only Walmart within the city of LA will not allow it because it's smack in the city and has a parking garage with limited height." They suggest casinos or further out camping/parking: casinos- www.casinocamper.com/rv-parking-locations
Free camping is available in rural areas:http://freecampsites.net/#!California&query=region
I hope this helps!