Every WalMart is different. Over many years we have from time to time stayed overnight in a WalMart Parking lot. We try to use common sense about where to park and in some Walmart lots there is very little room while other have acres of space. With our Monty we always have the bedroom slides out at night.
We have travels so many years that we have pretty much learned which WalMarts have space and which do not. As a courtesy to WalMart and others, we normally do not stop overnight in a Walmart lot that is small and/or crouded. The last one where we stopped overnight was the WalMart on I-25 near Fort Collins, CO. There was only one other RV there that night but about8 or ten semi-trucks parked. Plenty of room. A month ago we stayed overnight in the WalMart lot at Cody, WY. There were about 8 or 9 other RVs there that night and there was room for at least a dozen more.
We try to use commons sense when parking overnight at a Truck Stop too. Many are crowded with trucks so we rarely stay where we would take a truck spot or close in. Many truck stops have acres of space where even trucks rarely park. For example the last truck stop where we stayed overnight was the Truck Stop at the North edge of Lamar, Colorado on US 50/US 287. We parked along the back edge at least 50 yards from the nearest truck. Even with dozens of trucks in the lot overnight it was quiet where we parked.