I got a 10 gallon RV water tank on ebay for 25 bucks plus shipping. 10"X17"X21". But I sure like Madjacks tank system. The best way I know to make water last is to use a hand pump instead of electric. But I've been known to take a bath in the lid of my plastic bar soap dish when sailing off shore for any length of time. 2 of us go through about 40 - 50 liters of water in a week on the boat, but a lot of that was used rinsing salt off ourselves after swimming and diving for grub. If you can call a rock lobster and grub.
Steve in St Augustine