You have a 38 gallon waste tank and two 38 gallon grey tanks that may not be plumbed together. If the two grey tanks are not plumbed together your experience is not that out of the ordinary. It is very easy to generate 38 gallons of grey water in 2 days just by not being frugal with water when washing hands, brushing teeth and washing dishes. My 38 gallon grey tank will fill in 2 days with just my wife and I taking navy showers and normal use. The black tank could also be full in 2 days with 4 adults using it. Some will come and talk about going days/weeks with smaller tanks but your experience is more typical. BTW, my bathroom sink is plumbed to the grey tank and I believe that is not unusual.