The original post is kind of ambiguous as it sounds like you wanted to drop off your RV while you did shopping elsewhere. If you just planned to overnight and stay with the RV that is different. But it really is at the discretion of the manager or whoever has authority at the store. If they say "No" then you shrug and move on. Nothing really to get upset over. They are a retail store, not a campground.
OTOH, to me there is a difference between parking overnight, and dropping off your RV and leaving it to go get supplies from somewhere other than Wally World.