We've stayed overnight in WalMarts across the country. We always call ahead and ask. Then pick a spot as far out of the way as possible (sometimes over grass is not available) and put out our slide-out. No chairs and no grill. Of course, our Lance 1685 at 20'9" is not exactly a whopper taking a lot of space anyway. But the slideout always goes out. We also always go into WalMart and buy something ... even if we don't need anything.
The only "problem" we've ever had was on a Saturday night in a small town where apparently "the" place to hang out and cruise was the WalMart parking lot. It was a bit noisy for a few hours. :)