Working on the water tank. It's made from 1mm styrene and comfortably holds about a half gallon / 2 liters of water. The bottom is sloped to help the last bits of water find its way to the pump.
I want a large opening for maintenance access. Using a 2mm plate for the lid. I hope I'll be able to make it somewhat water tight.
The water level sensor drops down through a hole in the top. 5 wires connected directly to an arduino with its pins configured as INPUT_PULLUP. When the water reaches a powered sense wire a connection is made to ground and the arduino does whatever it needs to do.
Here it is in the truck. The plan is to add another tube to the top that will let air in and out, and a tube somewhere for the fill/drain pump.
Video of sensor, tank level lights and telemetry:
https://www.youtube.com/watch?v=VphOW5oXx9A