I kept my passenger as my "cargo" for food. Things tend to spoil VERY quickly in a car, because of the heat and direct sunlight, so you pretty much have to get food every other day. You also have to be careful about messy foods since you only have a limited supply of water (I used a beach-tub thing for water).
Fruits are good, bananas and stuff.
Bread is good. Crackers, Peanut Butter with bread.
Some cheese you can keep out, like string cheese.
I ate a lot of raw pepperoni. It lasts decently long without refridgeration but usually you just buy it and take it home and eat it that night.
I did eat out a fair amount, however.
It's hard to avoid: you can't really prepare anything because of size and water constraints, and you also might not have the tools or anyway to preserve foodstuff for long (and you have to clean the tools you do use after you're done). So you can have cold ramen or cold this or cold that but you can't really cook it. You might be able to find some sort of 9V tool that plugs into your cigarette lighter or find some sort of solar-oven or something crazy like that to help. My hobo-ness was unplanned and of an unknown duration so...
You could try to get powered milk with cereal? That might work out okay.
But I mostly restricted myself to packaged items (Gronla breakfast bars are great!), non-perishable items that you can eat with your hands. They mostly fell into a "snack" role, but I don't eat very much (1 meal + 1 "snack meal" is enough for me a day and sometimes if I go out to eat, I split my meal in half and just eat the second half that night). If you eat a full 3 meals a day, it will be more difficult for you.
