We also have been RV'ing more than 20 years and have spent a number of night in different retail lots around the country. I have to admit to never having seen any RV's parked for 'weeks or months' in any lot in our travels.
We always ask, spend a little money when we're there, never unhook, and always park where we're told or else where the tractor/trailers park, with only our bedroom slide out for the evening.
We have seen on a rare occasion RV'ers set up with their grill hot and chairs set around having a cold one, but I don't believe any comments from me would change their rude/inconsiderate ways.
It is a 'to each his own' world and we just try to slide along and get along. If Walmart doesn't welcome us then we'll either find another lot, find an RV park, or a rest area on the interstate.