Figure out where you're going to park and see if cops will harass you there. You may have to move a lot. = gas $
Figure out where you're going to bathe. Some restaurants have single stall bathrooms with faucets and locks others don't. You've got to watch the amount of time you're in there. People will complain and then they'll watch you every time they see you coming into the store. Some shelters will let you use the showers for free even if you're not going to stay there the night. Investigate that. Go visit. See how you feel about it. Use your gut and see if you get any bad vibes. You may just have to suck it up though if you get desperate enough. Make sure you keep your options available and that you know what they are. Don't feel like you're too good to take a whorebath out of a sink. It'll eventually happen. You'll have to.
Agree with SE about the cost of eating. It sucks. The healthiest foods need cold. Stock up on peanut better, bread, and if you can get a hold of a stove once or twice a week you can scramble up a bunch of eggs store them in a container and keep it on ice. The ice being replaced will be your most useful expense, food-wise. Get a GOOD storage container for your food and ice. Not Styrofoam. Ice is cheap.
Find shelters in your areas and soup kitchens. Go there to eat as much as you can and especially food pantries. Canned food sucks but it's food.
More about protein and fat- you need FAT!!! don't load yourself down with cheap carbs. That is one of the biggest downfalls of being homeless-- it's easier to get lean protein and carbs than fat and FAT is what you need to help you concentrate the most when in stressful situations. Stay the hell away from Spam at all cost. It's cancer in a can. Eat good fats. Eggs and such. Get a container of good natural guacamole and store that in your icebox. Eat a spoonful of that with your meal. PERFECT fat!!
If you can't get a stove or someplace to plug in a small single plate burner(those are worth investing in, trust me!!) then for fats and such I'd recommend learning to like pickled eggs. They don't keep, though, unless you stick them in the fridge so here again is where your ice will save you. Buy pickled eggs. Mash them up in a big bowl and stir in some mayonnaise(use restaurant packets) and there ya go. Egg sandwhich fixings. Keep your egg mixture in a container on ice and it'll keep for a few days.
You're going to have a hard time keeping fruits and such alive in a hot car. I always stuck with buying only a day or two's worth at a time. Lots of oranges and bananas. Things that don't need cold to stay fresh for more than twelve hours. Vitamin C is always a big deficiency when being homeless unless you do this. Of Course splurge on the occasional peach and whatnot when you plan to eat it that day but for stuff that'll last, stick with oranges and bananas for your vitamin C and potassium.
Oh! And broccoli!!! You can go to a good discount store(like Aldi or whatever you have close) and pick up two big bunches of raw broccoli for really cheap. Wash it, pat it dry, and then store that in your ice chest. It's really good for you to eat a few sprigs off of it with your eggs in the morning. I still do that, actually. Broccoli is the best source of vitamins, for sure. Yeah, it'll be raw but it's the best that you'll be able to do for yourself. Eat the fewest canned foods possible. That shit is deficient in every way!!! You'll get sick fast if that's all you live on. Fresh is best. With an ice chest you CAN do it!! 
I know this sounds horrible, but the summer time and early fall is really the BEST time to be homeless or living out of your car. Good produce is cheaper and you can stay healthier.
Laundry... laundry... Yes, you're going to spend some money on laundry every week, however there is a way to make it healthier and cheaper for yourself. Here's my dirt cheap recipe for laundry soap: http://www.terrascents.blogspot.com/2010/01/saving-lot-of-money-on-laundry-soap.html
I know you're not going to have a lot of room in your car but you do need to invest in some big plastic nesting bowls to fix things out of. Like soap. Or food. Whatever.
You can make a bunch of bags of this soap for dirt cheap(I breakdown how much you'll get from each box of ingredients there in the blog post), put it in ziplock baggies(one per batch) and then there's your soap for six months! If you can't shred the soap as finely as I do, like in the blogpost, then what you can do is to think ahead a big. Put as many spoonfuls of the soap mixture in a lidded container, add water, and leave it in the sun in your car for a few hours. Stir it around and voila! Melted soap. It's perfect. Just portion it out between the washers for how many loads you're doing.
Don't buy Tide or whatever unless you're desperate. It'll eat up your money like crazy. Look at the math I showed and you'll see how.
You do want to keep a big jug of distilled vinegar with you, as well. Great for washing hands, hair, deodorizing your car, etc. Mix a 1:4 ratio with water in a spritz bottle($.74 at Walmart) and you can freshen up your car fast and easy. Trust me. You're going to want to do this every day. You can even put a drop or two of your EO's for your hair in that spray mixture and use that to spritz your stuff down.
Use about a half a cup of vinegar in with your laundry, too. It'll help the handmade soap do it's job with your clothes.
Keep a big gallon or two of water on hand(refill it at any bathroom you stop into whenever you need) for washing yourself or anything else, utensils and whatnot. And the vinegar. Do NOT forget the vinegar!! Water doesn't wash. Vinegar kills just about anything. Stay healthy.
Umm... can't think of anything else right now. I'll keep thinking about this.
Been there. Done it. And these are the things I would have done better if I had to do it again.
That lock rocks.
