Honestly, you could do this. I am not saying it would be the BEST IDEA, but this is doable. First of all, I don't agree with Jamie; staying in nice neighborhoods will not work. People don't like looking out their windows and seeing mysterious vans parked across the street, especially when all the other cars are Audis and Mercs and shit. It's just ludicrously conspicuous. Buuuut I was talking to Steel about some other options and came up with a few. One is you could find an apartment building/complex, one where cars are parked all over and no one knows which cars belong to whom or, for that matter, really knows anyone at all. At this point you'd be able to get away with parking there and no one would even know you weren't supposed to. People probably wouldn't notice or report you, and cops don't really randomly cruise around apartment building parking lots so you're probably safe from them.
Another idea is to stay in a retail type area, but not in the parking lot. Parking lots are dumb because they're wide open spaces that are completely empty after the stores close. You're going to catch anyone who drives by's eye by being a LONE VAN in an otherwise empty parking lot in the middle of the night. Cops, thugs, all types of dicks! It's a bad idea. But if you've ever walked behind a shopping plaza, you'll notice that cars are parked there overnight all the time. These cars, I assume, are supposed to be there. But the point is if you're going to be anywhere in a parking lot, it should be here I think. For one, you're not immediately visible, so people driving by won't be able to see you from the road, or without actually looking for you. You're in a secluded area that generally doesn't get much traffic at night where ideally you wouldn't be noticed by anyone. Now, there are also some factors here that I didn't really figure out because I've never needed to put thought into this. There would have to be cameras in the back of these buildings. I doubt anyone is watching them consistently but they might notice a random van that comes there every night. Or maybe, if there were enough cars that stayed there overnight, they wouldn't and you'd just blend in with the random vehicles that for some reason people leave there. It's kind of risky, though! It also makes you look a bit more suspicious because this is where all the back doors to places are (another consequence of this is that people unload shit here in the early mornings and will see your van and possibly know it does not belong). This is, I think, the best of the parking lot options, though.
Another idea is a parking garage. I'm not sure but these seem like they'd be relatively safe and cops don't just DRIVE THROUGH THEM do they? In any case, you'd pay admittance and probably be allowed to stay there in relative seclusion through the night. On one hand, parking garages have security measures in place designed to protect cars that park there, so that would in theory contribute to your safety, but a) I don't know how thorough or reliable those features really are and b) I don't know if they'd really let someone sleep there on a regular basis.
I think the best idea if you try to do this is to find a friend and try to get them to let you park in their driveway at night. You must know people with HOUSES, which means maybe you could talk them into letting you just park there for the night and have a safe place to sleep.
Finally you could maybe try a storage space. You can get storage spaces for cheap as hell and with one big enough, you could actually park your van in it overnight. With some lighting, you could probably even live in the storage space itself! They're not heated, which would be a big deal here, but in SoCal I don't imagine it ever gets very cold, even at night, so you might be alright. It's a secure place that you'd have a legal right to, though, and it's cheap. You could either park your van in it and just stay in the van at night or get a big one and park your van in it and then get out and have like a bed and a desk and shit in the storage space itself. The owner wouldn't want you doing this but there'd probably be ways around this. This is probably the SAFEST option as you'd be completely protected and no one would even know you were there.