You don't, you ask at each store if it's permitted, and if so, where you should park.
Well stated Ned. We always ask, and we've had the service desk people suggest we not stay for a variety of reaons including recent crime, bad neighborhoods nearby but not seen, ect... If they say they wouldn't do it, you can bet we move on.
Our worse experience was in a rest area with a state police office in the main building. Over nighting was allowed, but we lost a gas cap and almost the spare tire. It pays to travel with a reasonably large dog. She surprized the daylights out of them and I gained a pry bar for my collection.
Our only bad WalMart experience was in winter in Vermont. The coach battery died and the -20 degree temp combined with the 10-15 MPH wind froze us.
Can't bame WalMart for that one.
Joe