If we are pulling in late I’ll call ahead, ask permission to put “permission granted for dates x to x overnight parking, via phone conversation at x time.” I write the manager’s name. We put it on our dash or affix to our driver side window. We go into the store, talk to the manager, and have them write permission on a 3×5 card I supply.
I’ve lost track of how many Walmart parking lots we have spent the night in with our RV from coast to coast and north to south.