I picked up our van from storage at AndeanRoads motorhomes a couple of days ago and their rigs are all fitted with cassette toilets. From my research it seems as if most "camping" down south is proper camping with tents and there are very few facilities for dealing with large holding tanks.
Some areas of Argentina have problems with water quality during the dry season and regardless of what the locals think of the water, you might have problems dealing with it just as sometimes happens back home. Chile claims to have good quality water everywhere.
We just use whatever water comes out of the taps and if there are doubts about contamination then we add small quantities of common bleach as we fill the tank. The only bottled water we buy is one gallon for emergencies and that invariably sits in the cupboard forever.