For us the black tank is the one that runs (so to speak) out of capacity first. This is compounded by the shower drain going into the black. I'm with Rutz on this one. With me doing my #1 business outside as much as possible, we can go at least 7 days, maybe 9 days, off grid and off road with these tanks:
fresh: 18 gallons
black: 14 gallons
grey: 12 gallons
We host 24 bottles of waters in the space in front of the wheel wells, and another 24 bottles on the floor behind the front seats.
We host 5 gallons of fresh water in sturdy gallon jugs (not milk jugs) in a cabinet on the floor near the door.
We wash the dishes in a plastic pan in the sink and dump the grey water outside away from camp.
We take military grade showers sitting on the pot with another plastic tub on the floor to catch most of the water that would go down the black drain, dumping it outside after use.
The upside is that you don't have to carry all that black and grey liquid/semi-liquid with you.
We have a pop up shower enclosure for using the outside shower. Works great.
We have learned to live with small tanks and that carries a certain satisfaction.
jefe