It is a bit more complicated....
Walmart's corporate policy has always been to allow RVs, trucks, campervans and travelers to park and sleep in their lots. There is nothing whatever illegal or against the law about it. However, Walmart allows each individual manager to set the parking policy for each store, and since most Walmarts change managers like we change socks, the policy often changes at random, from one direction to the other. So a store that allows parking one week can change the next, and vice versa. But it is then up to the store manager to inform customers if the store does not follow Walmart's corporate policy. (And alas the various websites that list Walmart parking are usually way out of date and unable to keep up with the constant changes.)
There are also many Walmarts in which the official policy is set to "no parking" but which in reality allow parking anyway. Most times, this is because the local jurisdiction has an ordinance against "sleeping in a vehicle" and makes Walmart put up the signs, but the Walmart itself has no objection to people parking and won't enforce the ordinance. (And indeed most Walmarts do not have any security personnel in the lot and make no effort at all to enforce any such ordinance.) In many other cases, it is because Walmart itself does not own the parking lot and cannot legally give permission to park there--but again they don't actually object to it and won't care if you do. So the vast majority of Walmarts who have "no parking" signs up actually don't mind if people park overnight, and won't ask you to leave even though it might be technically banned. I have parked in many many of these.
Even in many Walmarts that do have a "no parking" policy and who will enforce it with security and actively ask folks to leave, we traveling vandwellers are not their target. Despite the rather silly conspiracy theories that some people have about Walmart colluding with "Big RV Lots!" to "force people to pay to park!", any time a Walmart bans parking in its lot they are always excruciatingly clear about the reasons why and make no secret of it, and those are always the same reasons: homeless people who move in and stay for years, homeless people who make a mess and dump their shit (literally) in the parking lot, and homeless people who panhandle all the customers. If you are not one of the morons who are doing those things, most Walmarts will simply not bother you. When I park, I am a good neighbor. I do not make a mess. I do not attract any attention. I do not make any light or noise. I do not look like a homeless person. I stay in the back of the van, unnoticed and overlooked. I move the van several times a day and never park in the same spot. Virtually none of the customers or employees ever even notice I am there. Several times I have had Walmart security folks wave to me as I walked across their parking lot, so they obviously knew I was there (and indeed it is their job to know I am there). But since I was not doing anything they objected to and not causing any trouble, they left me alone. I am not the droids they are looking for.
