One artist who also had a 32 ft. boat kept all his water in 2 liter jugs under the settees. Did not have tanks
in his boat. But it was easy to keep track of how much they used.
Since your water is in tanks, if you measure how much it takes to fill the tank, and log the date of filling and when you run out, you'll also be able to come up with an average usage figure. That will let you plan your watering up stops, You might want to carry a 20 liter spare water supply for emergencies.
Measuring your usage as you use it also helps make you aware of how you use it. You can make it almost a game
to see how little you can get by using. It really makes you notice how much water landlubbers routinely waste! Another water-awareness trick is to get it in 5 gal. jugs. and carry them to the boat, and keep on doing it till the tanks are full. It all helps you transition your thinking from land use to your tanks being your own finite well, as it were.