Truck stops are the safest place they can stay as there are other people around and at night there are usually security guards as well (they serve mostly to keep the sex workers from knocking on the truck cabs and waking up the drivers). They also have CCRV of the area.
Wal-Mart parking lots are the very last place I would stay and be safe. They are usually in the low rent part of town and you are out by yourself.
Rest stops along the highway are another option. We find an RV park and pay the $40 to get electricity and a hot shower. If we could not afford the $40 we would stay at home.