We've stayed at many Walmarts over the years. Don't always ask permission. However, I look for other RV's parked around the perimeter. When I see that, I just find a spot and park for the night.
According to the posted map, the Barstow Walmart does not allow over night parking. Not sure if things have changed recently, but we stayed there one night about two years ago. We were told by security that we couldn't park there. I checked with the store manager and was told it was fine but to leave early.