Walmart actually sells a road Atlas with all the Walmart stores marked on it for the convenience of campers. We've stayed overnight at many Walmart stores. It depends on the purpose of our trip -- if we are on the way to a specific destination rather than just relaxing in the RV, we will stay anywhere - we've spent the night in Walmart lots, truck stops, Interstate rest areas, university parking lots, and once the Chamber of Commerce parking lot in downtown Orlando. In fact, we've probably spent more nights outside of traditional RV campsites than we have inside them.
Obviously, these are just stops for convenience, not for enjoyment. One does not stop at Walmart and open the awnings, set out the lawn chairs and portable firepit, etc. It's simply pull into the lot, set the levelers, close the curtains and sleep. And, of course, you must have a self-contained rig with generator, holding tanks, etc.
Generally, all of the 24 hour Walmarts cooperate. We used to go see the mananger, but discovered that most of the 24 hour Walmarts have a roving security person who knows the ropes. If he doesn't chase you away (and that's never happened to us), we're good to go. It's nice knowing that someone is wandering around the parking lot all night and won't let anyone bother you.
I think the record that we have seen is 12 RV's in a Walmart lot near an interstate highway. Of course, we return the favor by stocking up on needed supplies while we're there, usually groceries, but occasionally a spare part from their relatively good supply of RV equipment. My wife is a quilter, and is certain that every Walmart has one of the pieces of fabric she is looking for marked down to $1/yard, so she has to check them all out.
It's much more comfortable than being sandwiched between two semis with diesel reefer units running all night at an interstate rest area, and your neighbors are not as nosy as they are at an RV campsite. I guess it would be accurate to say that we are not traditional RV'ers -- we use the motor home as a tool, not as an end to itself.