I installed a fresh water gauge. Works well. A word of caution though. If you put a sensor (float type) in the water tank to drive the gauge, then you are not going to want to use vinegar to clean that tank in the future. It seems that the outgassinig (fumes left in the tank) from the vinegar will corrode the contacts on those sending units and you will end up replacing it. Ask me how I know.
I installed a Snake River Systems tank monitor system many years ago, and it works perfectly. It's a four-channel system, 'though I only installed it for two water tanks. It's a capacitive system, nothing inside the tank, just strips of aluminum on the outside.