Don't expect to get much sleep when you are trying to sleep in your car. Comfort is not there. But, use a truck stop (Love's, Pilot, Flyin' J, T/A, Petro, or any large one) and park in a comfortable spot that does have occasional to heavy foot-traffic nearby. You can trust the truckers. It is the other nut cases you will have to worry about. Just be courteous.
In the old days you could park at a small truck stop and leave a wake-up call at the fuel island. Try that anywhere today and they will call the Nut Farm to come and get you.