As we are travelers more than campers, we overnight in lots of places that are not thought of as camp grounds.
When we have had the opportunity to ask, of the so many times, I think we have been refused about twice. For us, as many of the places are retail we ask as "when we finish our shopping here, can we spent the night in your lot?" We are seldom refused. I also very specifically remember that once we were refused with that request, we also figured out that the new stop should have what we planned to buy there. That sounds fair - doesn't it.
Matt