You would probably find out from the service desk, if you ask, that it is a city or town ordinance ruling and not the Walmart.
I have the app “park advisor”. It lists tons of places like Walmart’s etc and when you click on the info icon it tells you if that walmart allows rv’s. It also shows city parks, rest stops etc that allow over night stays. Also has contact info for CG’s. Pretty decent free app.
You might look into Harvest Hosts. Small annual fee and you can stay at vineyards, farm markets, small museums, and specialty farms for no cost. You’ll have to dry camp but it sure beats a Walmart parking lot. Please note that you are encouraged to patronize the business but we’ve always found that to be part of the fun.
There are some campgrounds that do let you park in their overflow area at a reduced cost. You can also stay at truck stops and rest areas. I use RV Parky and Good Sam and can usually find a campsite for the night but have had to stay in overflow or truck stops too