Unfortunately, it is strictly prohibited to park overnight in the parking lots of shopping centres and rest stops. Walmarts are known to permit overnight parking on their store lots, but be sure to check in and ask the store manager for permission first. If you need a break to stretch your legs, rest stops along the highways can be used for up to 4 hours at a time.
For more information on RV travel in Canada, the Authentik Canada website has a good overview.