We do it all the time when we are travelling from one place to another. We always need groceries or whatever and certainly do not need a full hookup campground for an 8 - 12 hour overnight stop. The store always profits from our stay and we never leave a mess or take up parking spaces near the entrances. It is a win-win situation. And yes, we put out the slide and have been known to set up the dish in "stealth" mode (between the MH and tow car where it is not noticeable).
I just checked and see we spent $1,600 at Walmart last winter. Almost all of this was because we were parked there anyway as Walmart is not normally our preferred store for shopping.