While it may be cheap to spend the night at a rest area, it isn't necessarily safe. Many states have banned sleeping at rest stops due to increased crime, and many others have put up signs that discourage it. Your best bet is to look for campgrounds or state parks along your route where, for a small fee, you can more safely snooze in your car. If you have to sleep at a rest area, in an RV or car, keep the doors locked and don't open them to strangers. Talk to any strangers through the window or door, and if you feel threatened, drive away.