There are two WMs within an hour of our current location. While I was running errands, I stopped in both of them the same afternoon, and asked the manager about RV parking on their properties.
One is in a high traffic area, and their lot is relatively small. The manager denied permission to Park overnight, saying that the trucks coming to deliver goods take up most of their available space, and he could not allow overnight parking except for trucks delivering goods.
The second store has a very big lot, and seems to have plenty of room. The manager there told me that they allow overnight parking as long as RV's do not interfere with delivery traffic.
Both parking lots have signs stating "no overnight parking." These signs are "CYA" signs if the manager finds it necessary to call authorities to evict a camper.
So, again, we are back to "Ask store management for permission."
Seems simple enough.