We prefer campgrounds as well.
However, if we are just wanting to get some a "shuteye", casino's are an upgrade from parking lots. Most casinos welcome RV's and have decent security. They're hoping you will come inside and spend money. Even though it is not required.
Many also put trucks in a different location, so it is much quieter. Truck stops are just too noisy! (Unless I am desperate)
Don't know about the country as a whole, but casino's are all over the place in the southwest.
Regarding parking lots, just use you head. If you don't feel safe don't stay.