@Bast: The (un)availability of food should be the last of your concerns. I spend about 80 dollars a month on food and eat quite well.
Even if you can't raise 80 dollars in a month by hook or by crook, there are numerous private and public charities that you can take advantage of. My younger brother who is chronically short of money goes to this church who regularly gives him far more food than he can eat by himself. In many towns there are numerous food banks. If you get tired of eating canned foods many churches and shelters serve hot meals to the public.
If you want to eat really well for nothing look into volunteer internships on organic farms. You won't make any money (with a few exceptions), but if you choose your farm wisely you will eat better and healthier than you ever have before. A great stress reliever after the stress of trying to van dwell in an urban setting.
On a different note, right now car dwelling here in eastern NC my monthly expenses look to be around 300 the next month (excluding college tuition). I can tell you, it doesn't feel as great when you have no choice but to live on a shoestring, but currently my finances are looking great.