We've used Walmart as an occasional overnighter in both Canada and the U.S. Never had any issues other than noise. We always ask first, as they often have a preference as to which part of the lot you park in. One morning, in Michigan IIRC, we woke up and there were >30 RVs in the lot, there was even a tent setup on the asphalt next to our trailer!
Unlike some who I'm sure do not frequent this site, just make sure to keep the area clean and respect that Walmart just hosted you for free. I've seen some campers putting garbage bags beside Walmart garbage cans. I'm sure Walmart staff just LOVE to see RV garbage piled up by their front door!