Here's a no-brainer if you have visual access to your water tank and you can see the water level through the sides.
Start with it empty and add some convenient amount of water. I have a 25 gallon tank so I added 5 gallons at a time. Mark the level then add some more. A quick pop of the side storage door gives me an idea of whether I have to make a water run while boon docking. Not a biggie.
I travel with only 5 gallons in the water tank and when filling before a trip with the water hose this gives me a guide. (Trivia: Did you know that the Casita
16 is longer behind the axle
than the Casita
17'. Makes water behind the axle
a bit more of an issue due to the lever arm.)
There are water level sensors but for my needs this was sufficient.