Walmart: I was in a semi-local Walmart, returning something at the Return/Info counter, and ahead of me was a man carrying a couple bags of his purchases, asking about permission to park his RV for a couple of nights. The clerk gave him permission. In the parking lot, he was parked only a couple of spaces past me. Gathering info for my future travels, I walked over and asked if he has problems with permission. He said that many of the WMs had standard rules, yes or no, but some of them were more agreeable if you did your shopping there, and then asked on your way out. He said he might be wrong, but they seemed to have more positive results if they proved they were shopping there, and weren't just taking advantage.
It might vary a lot, but it's something to keep in mind.