It is okay to sleep in a car as long as the weather permits, I have done that a couple times myself once in a rest area and other in a Walmart. I was travelling from Minneapolis to Washington DC but the weather was great, I had a comforter and a sleeping bag and slept in the rear seat, but at walmart, you will have to contact the staff and take permission from them (some Walmart's do not allow you to sleep in their parking lots, but there are only 20% that do not allow you to sleep in your car).
Precaution to take:
- At a rest area, you will have to make sure that there are people in trucks and are parking for the night ( Truckers are good guys).
- If you want to run your car roll all the windows down a little bit, but what are you gonna get out of it, if you are burning fuel, better find a cheap motel.
- If at a Walmart make sure you are below a light post or as close as possible to the store's entrance.
- Make sure the space is properly ventilated and lock your car, though your are inside the car.
- Stay in well lit areas.
Here are some links that might help you:
Walmart parking lots where you cannot park: http://www.allstays.com/c/walmart-locations-noparking.htm
A Guide to Car-Camping – in Walmart Parking Lots: http://themorningfresh.com/2012/06/29/a-guide-to-car-camping-in-walmart-parking-lots/
There are a lot of people out there who made their car as a home.
Watch for videos on youtube.
Good Luck and hope this helps you out and stay safe.
Don't forget to share your experiences.