I have always preferred mechanical devices rather than electronic for water sensors. In my case ('06 Monterey), the physical water tank is translucent, so I just took a magic marker and marked the outside of the tank in 10 gallon increments on the side of the tank. That way, a quick opening of the bay door and I can see exactly how much water I have left. On my previous coach, the tank was ABS black plastic, so I could not see through it. I added a small fitting on the bottom of the tank with a clear plastic tube attached up the side of the tank with gallon markings on the clear plastic tube. Since water seeks its own level, I could always tell how much water I had left by looking at the clear plastic tube. Where there is a will, there is a way.
Richard
p.s. On the ABS plastic gray and black tanks, I would often use a lazar temperature gun on the outside of the tank. The tank would be cooler where there was liquid on the inside.