I can't speak for every state as truly I don't know... But Michigan I do know.
The parking lot of a Wal-Mart is PRIVATE PROPERTY, now, police can enter private property w/o permission of the owner or his authorized agent (in this case the store manager) for a number of reasons.
1: If they are shopping wal-mart
2: If they are in "hot pursuit" of someone who runs across said property
3: If they observe a Felony in progress
4: If they observe a fire on the property
5: Only as far as is required to request permission
7: To perform a property inspection for ordance compliance (This however affects the property owner, not you the parker)
8: In some cases handicapped parking rules are well written and allow the officer to ticket on private property
9: With a warrant
Oh yes, there are other reasons not listed here, but basically not one of them apply to a parking violation
So, the only way, here in Michigan, they could ticket you at 3am on a Sunday would be to get a judge out of bed to sign a warrant. For a parking ticket, likely about $100... Trust me.. They are not going to do that... (If they tried it might well be 3am NEXT Sunday monring, long after you have moved on, that they get back to that parking lot)
They can not enforce parking regulations on private property WITHOUT THE Store Manager Requesting they do so.
They also can not enforce stop signs and other traffic regulations on private property without the manager's request
The greatest danger would be some jerk parks a big SUV right in front of the store's front doors, and the manager calls to get him ticketed or towed... And the officer takes the invite to include your rig... In my years of working in retail, I have observed many such jerks parked right in front of the store, in the very clearly marked fire lane... NOT ONE TIME did the manager call. In my years (25) as a police dispatcher, We only got calls if the car was there for many hours (Like 2 or 3 days) or if the owner of the car was someone the store owner wished to run off (this happened fairly often) In my 3 years as a tow truck dispatcher the illegal parker has to be very bold in order to get towed.
And finally... if your rig is properly parked, and you don't drop the jacks or put out the awning or lawn chairs... How is the officer to know you are not just shopping... I mean.. If I was overnighting at a Wal-Mart I would put out one slide (the bed) but If I was buying Toilet Paper I'd put that slide out as well cause the main bathroom is on the far side of the bed and it's either put out the slide or climb over
(NOTE: in all cases another employee can step in for the manger,,, However with wall mart the employees serve at the pleasure of the next person up the corporate ladder, the top rung at the store is the on-duty manager and any employee who called the cops on you (other than he) would be in a world of unplesentness when the manager found out)
Now... A passing campground manager can write down your license plate... but only the store manager can request enforcement
Of course the city can fine Wal-Mart for allowing ... but how many ordiances are written to allow that? (Likely zero)
Let me see if I can get lucky with Detroit (Alas, I'm on the cell connection at this time and it's not fast enough to load the Municiple Codes.. Will have to grab them when I'm on DSL) Of course there are ano wall Marts in Detroit at this time
I know Detroit has an ordance against a campsight within 10 feet of the curb (or some other arburtary distance) as I recall reading it on a trip to find out just what ordance they claimed I had violated once (Turns out I was not in violation)