Future van dweller here, still in the dreaming stages. I can't speak for the city of San Diego specifically, but I can speak for the north county coastal areas (Cardiff, Solana Beach, Encinitas, Leucadia, Carlsbad, (maybe Oceanside but it's a little dicey there). I've been thinking and planning this for over a year and trust me, if you take time to research areas, there are TONS of places you can stealth without any problem. I suspect this is the case anywhere, you just have to look closely.
Hell, I see a handful of people in RV's parking lot hopping all the time. Had I not been thinking in terms of van dwelling, I likely would have never noticed them.
EDIT: In my opinion, the coastal areas are probably a better bet for stealthing because you have a higher concentration of people in a small area. Although I'm not aware of any place you can park on PCH overnight, there are many side streets with many cars parked on the street. In some areas, 60+% are vacation rentals so there's a constant turnover of different cars thus making stealthing even less suspicious.
