Beamalarm.com has a list of all the rest stops by state and whether they allow parking overnight. I did it for several nights on a trip in my RV from Denver, Co to South Florida several years ago and it was fine. In the states that allow it, you will almost always find a few RVs together around dark. It was nice on the interstate not to have to stop early and look for a site. I drove until just before dark and stopped at rest stops until I found a few RVs parked for the night.