We spent a short night at a Walmart in Rifle Colorado. I didn't ask I just pulled in and went to sleep. I was in a Truck Camper. But usually if on the road and its getting late I will google the nearest walmart and call them. I find most of them in the smaller town areas are fine with it. Our local Walmart has signs but people over night every night in the camping season. I think a lot of those signs are ignored by the management and the police. I think they are there in case there is a problem. JMHO
I generally like to find a city park or state park and pay the fee. I do not sleep the best in a Walmart parking lot. But sometimes it comes in pretty handy!