Some Walmarts allow folks to park in their parking lots overnight. Some do not. Boondocking in a Walmart parking lot is no one's first choice but it is important we camp appropriately so we may retain that choice. Wallydocking has been a blessing to many a tired driver or folks without a campsite reservation.
Other places with similar policies include Cracker Barrel, Bass Pro1) / Cabela's, and many truck stops.
Best practices:
be respectful; it is their property
obey any signs that say overnight camping is forbidden
ask the manager if it is ok
if you need anything, purchase it at that location. Holding your purchase while you talk to the manager in the previous step is a great first impression. Eat at Cracker Barrel. Get your fuel at the truck stop.
stay only a night or two
park away from trafficked areas; at the back/sides/garden center are usually good.
make no messes, and pick up any messes you see
maintain a low profile
keep your slides in
jacks up (or on pads if absolutely necessary); we don't want to damage the lot
no generators
no lawn chairs or other gear outside
for doG's sake do not drain anything in the lot