Since most KOAs appear to be privately owned, suspect it is up to each owner. Now some I have stayed at don't have a monthly rate, they are happy for a person to stay a month or more, such as Ouray Colorado. The KOA just north in Montrose Colorado is heavily into longer term stays and has a separate part of the CG set up for them, including on site propane that is metered. Several we have stayed at have separate areas for short term and long term, some even have separate roads out of the park for those long term campers as many get up to go to work daily and leave early.
Just by the locations of many/most KOAs I would suspect that short term, such as overnight, stays was in the planning of the franchise. Not too many are what I would call destination campgrounds, where some are, such as Mount Rushmore KOA, Ouray KOA, etc.
