Kay, (Daniels, WV)
Some large coaches actually have three holding tanks, especially those equipped with a washer/dryer. My thinking is that if a motorhome only had one tank, it would fill rather quickly since the draining sinks and showers would fill the single tank rapidly; unless that one tank is quite large. And therein lies the most probable second reason for separate holding tanks. It’s easier to mount and design a motorhome around two smaller tanks that one large super tank!