We have a truck camper. Fresh Water is 9 gallons, but I have an additional 6 in hot water heater. Grey is 6 gallons. We can get 3 showers with little issues by taking Navy showers. But if I wash my (long, curly, thick) hair I need to use a pot to catch/conserve water.
The capacities you all are tossing around are LUXURY! LOL
Maybe it wasn't the shower part that was the issue. Did you run water while waiting for it to get warm? Did you flush toilet repeatedly? Waste water doing dishes? Let it run while brushing teeth?
One newbie who doesn't understand boondocking/water conservation can screw up an entire trip really quickly....
EDIT: The oxygenetics showerhead is the key to conserving water and still having a relaxing shower.