In most cases WalMart has no problem with RV parking.. The exception is the small "Neighborhood" stores you find in some places.
HOWEVER there is a "Campground Owner's Assoication" which sees RV's parked in WalMart lots and they think "We are losing out on profit here" so they lobby the city, county or state to prohibit overnight parking of RVs (And in some cases Semi Trucks) anywhere save a licensed campground.
And as noted above, occasionally you get an idiot who ruins a store for us.
Many stores now worry about liability since WalMart got sued by one of those idiots who should have known better than to let a drunk into his rig, YES< he opened the door himself. (OH, he lost the suit, but WalMart had to spend money to defend). Other stores heard about that and closed their lots.
But usually it is state, county or city laws/ordinances/rules that prohibit overnight parking.