Of the thousands of Wal-Mart stores, there are about 350 of them where you cannot park your RV there overnight. Mostly these restrictions are due to local ordinances, or in some cases due to the conduct of past RV'ers themselves where the store manager has had to eliminate the practice.
Many of the local ordinances restrictions are in Florida. (Arizona also has many restrictions).
For example, you may not park your rig overnight at the Super Wal-Mart on hwy 192, about 10 miles east of WDW.
You should always check with the Wal-Mart manager prior to staying there to be certain that it is allowed at that store. If you park where it is not allowed, you may be woken up by the local police at 3:00 am and told to move, or worse, be given a citation.