The way I see it, literally, in my local Walmarts is that if you stop by in the evening on your way to a destination, ask permission if you can spend the night, then leave in the morning, no problem...
But, if you are there for over 24 hours, you are homeless and basically taking advantage of the offer and ruining it for those that do just need to spend maybe up to 8 hours to rest between destinations...
I've seen rigs parked in my local Walmart for a MONTH.. No idea how or why they allowed that, but that was a couple of years ago and I have not seen that since, so I'm sure they at least stopped folks from doing that..
If you want to only stay for around 8 or so hours and you need to put your slide out, I don't see a problem with that... Just be gone by the next time I visit that Walmart.. :)
"Camping" is not really what the intention is.
I've never done this in a Walmart before, but just from being a customer to my local Walmart, I notice the rigs that are there "camping" and those that are just there to get a few hours of rest....
Mitch