Over nighters in waysides, truck stops, and Wal-mart parking lots are generally pretty safe. Tell them to pay attention to their surroundings and use their "spidey sense". That's one of the things international travelers are told. If it doesn't feel right, it probably isn't. Look for places where other RVs and TTs are. Avoid the waysides in Georgia and Florida after dark. Very strange places.
Regarding Wal-Marts, Cabelas, etc., often if you can't park there it's not because management won't allow it, it's because the local municipality doesn't allow it. Call ahead or stop in and ask a manger (not a general employee!)