Most central MA Walmarts don't allow overnight parking now. So I'd highly advise looking for any signs that may so no overnight parking.. I actually got woken up in the middle of the night once by a cop who asked me to move across the street to the truck stop. This was the leominster Walmart, I was in a U-Haul moving. There was only one sign posted...

But I've seen no overnight parking at almost every Walmart I go to now. You're better off finding a truck stop to park at, not only is it much safer(sort of), you also have amenities available. Plus less likely to have people screw with you or cops.