Some Walmarts welcome RV parking. Some don’t. Some Walmarts have large easy access parking lots with plenty of RV room. Some would have traffic flow issues if more than 2 or 3 RVs showed up at the same time. There are 4 Walmarts within about 10 mile radius of where I live. One has a large easy access parking lot to handle several RVs. Another also has a large parking lot but stays so busy that RVs would cause problems in the parking lot. And two have parking lots that are not only busy but not deigned for easily getting large RVs in, around, back out.
Assuming the city allows it, it is a manager decision, not a corporate one. Don’t worry. You will not be missed.