Who are you asking about etiquette? Surely don't ask the people that live near a WM or parking lot that others feel they can use as a camp ground! If a trailer is parked and the folks are in shopping,do they deploy the slides?
If your slides are out, whet next? Next guy pulls in sees that its ok to camp there,, his awning goes out. then The local shopping center has been transformed into a gypsy gathering!
Proper etiquette would be to spend the night in a camp ground designed for your trailer,fifth wheel,motor home etc!
This practice was once a courtesy afforded by WM but got out of hand. Try deploying the slide outs at a highway rest stop,, bet you highway patrol doesn't allow it.
I doubt that for the folks that live next to a walmart that my trailer in the lot is their biggest concern. You obviously dont like to overnight at a WM so dont. No need to ramble on to the extreme of it becoming a gypsy camp LOL. Also I have had my slides out over the grass at many highway rest area and never had a problem with the highway patrol.