There are very few Walmarts quiet enough for the Tail Vail. That's just too much temptation for the knuckleheads that like to mess with people overnighting in the parking lot. Without being able to stake it down, it will be flapping in the wind. We do extend our bedroom slide when we are at Walmart. It's curbside, so it doesn't interfere with anything.
We've slept in the back of our Astro Minivan at a Flying J once when the kids were very little. I didn't get much sleep, way too uncomfortable for me, but everyone else seemed to do okay. I felt pretty safe in the minivan with everything locked up. I would not feel safe with the hatch wide open.
-Michael