I use an app called ALL-STAYS for locating the Walmart's in Canada and the US. It's FREE. You can also purchase the same app ... ALL-STAYS Truck & Travel for $2.99 which includes Flying J, Pilot, Love, Home Depot, Lowe's, Service Centres, rest areas etc. etc. etc. I use both and they are great.
As previous posters have stated it is always best to pop into the store or call ahead and confirm you can stay. As I always remind people these are overnight stays and NOT campgrounds. Don't abuse the privilege extended to the RV traveller, by setting up a camp site with awnings extended, BBQ's setup, lawn chairs etc. It happens and as as result we are losing the privilege at several Walmart location throughout Canada and the US ... for a variety of reasons.
There is also another option as well. BOONDOCKERS WELCOME is a site where fellow members offer a FREE parking spot on their property for the passing RVers. I have joined this service and have used it several times now. They currently have about 600 locations around CANADA and the US.
http://www.boondockerswelcome.com
Happy travels
Karsty