I know this is an old post, but if you're still wondering, or if anyone else is in this position, yes, you can sleep over in a Walmart lot. I know, I've done it some. As a matter of fact, we did just this while visiting our niece at the
Chrysalis School Montana A lot of truckers and RVer's park overnight there too. You shouldn't have a problem. Rest area are good too, but be careful there, it's a little more dangerous than a Walmart.
Neil