Here is the only thing the Walmart website says about overnight RV parking. Anything else is just someone's opinion.
"Question;
Can I park my RV at a Walmart store?
Answer;
While we do not offer electrical service or accommodations typically necessary for RV customers, Walmart values RV travelers and considers them among our best customers. Consequently, we do permit RV parking on our store lots as we are able. Permission to park is extended by individual store managers, based on availability of parking space and local laws. Please contact management in each store to ensure accommodations before parking your RV."