A drain field/septic system has a capacity. as the system starts to fail the capacity goes DOWN DOWN DOWN.. I dump up to 70 gallons if I have a traveling companion.. all at once.. and typically many RV's ..
Well last time I pulled in to a WATER/ELECTRIC and Dump station park after Church I rolled down my car window (RV was on site) and spoke to some of the folks in line. about 5th or 6th in line And the line went on for about 3 or 4 more rigs. around the corner and down the hill.
Imagine all that going into the drain/septic sysrem ar one rtine. Hundreds of gallons.
The 3 you saw.. Likly do nor output nearly that much nor do they all dump in a line..