Do all allow or just some.
Neighborhood Wal*marts, small stores that have "Business Hours" (not 24hours per day) fairly rare, Small lots no overnight parking.
Regular and Super Store Wal*Marts: With one excpetion do allow overnight parking by company policy however read on.
ONE store, Auto dealer next door has been a pain in the policy, put his excess inventory on WM's lot so they had to ban, Apologies to the RVers, but they had to ban.
MANY: the Campground Owners Association lobbies government bodies, City, County and State, trying to force us to pay their members for parking.. No matter if its 15 miles of hard driving and you are already over tired, No matter if it is often cheaper to park in a CG and eat "Home cooking" Than to eat in the restaurant attached to teh WM lot or Flying-J. They cite such things as sanitation (Helped by the idiot who dumped his tanks in a WM lot,, NOTE the use of the singular) and such to get governments to ban overnight RV parking except in a licensed campground.. THIS IS NOT WAL*MARTs FAULT.
Most "posted" Lots, are due to such laws.. I believe Nebraska is one such state by the way. Others I can not comment on as I've only encountered a state wide ban in NE. Clark County, NV has such a law as well but under pressure from the Casinos has modified it some.. Not sure the current status as I've not been back there since it's passage.