Earlier quoted context omitted.
Most properly sized and installed septic tanks do not ever need to be pumped out. They compost on their own and the excess water seeps out underground.
Actually, a proper septic tank (according to EU regulations) has to be water-tight and be emptied regularly. Many beaches on the Mediterranean loose their health pass occasionally due to nearby overbuilding and improper septic tanks.
RVs have holding tanks that must be emptied. Thus, they aren't called "septic tanks".
But I'm sure the term is used loosely.