Walmart stores do allow overnight RV parking in their parking lots in most locations.
However when traveling I've seen a few Walmart stores that said no overnight parking so I guess it's up to the individual Walmart location as to whether they will allow overnight RV parking.
I doubt you'll be bothered though at 24/7 Walmart if you sleep overnight and then immediately leave as soon as possible in the morning.
As long as you don't set the jacks out or lawn chairs etc you should be find to camp overnight at Walmart.