WE have done it with no problem. In fact the parking lot grade is often better than some of the campground sites we have pulled in.
In our case we typically call the Walmart ahead of time and ask permission. You can get the phone number off of the websites. We have never been turned down. All they ask is that you do not park up front at the doors, but welcome you to use the back parking lot. Why?? Because you are sure to go in the store to stretch you legs... and inevitably shop.
Some Flying J actually have designated spots for RV overnighting. They also offer free dumping station.
Just call ahead (courtesy) and enjoy.
GPG