I've mainly found that most big box stores or any store that is open 24 hours isn't usually a problem to park at. The main rule of thumb is if they don't allow overnight parking, just don't stick out and make it obvious you're camping. Staying in a vehicle that doesn't look like an RV helps out a lot as well as keeping quiet, I wouldn't attempt it in a full blown RV.
For walmarts that aren't 24 hours I've noticed they usually run a skeleton crew at night that parks off to the side. If you can find out where they park it's usually no problem to just pull in and sleep, walmarts have such high turnover it's doubtful everyone has memorized what everyone else working there is driving. With that said, there's usually way better options than big box stores for overnighting with a little research (freecampsites.net included)
