My last boat, an '04 Nautique 210 TE, had a very similar issue. I never got mine fixed, and as Wade stated, I could always tell when it was full by the overflow. It's not the right way to do it, but it's the easiest and cheapest way. Seems like every time I took my boat in for one small issue, there was something else popping up. I want to say it was due to a faulty float in the tank though. Sometimes the gauge floats (or whatever you call them) get stuck and therefore don't allow the dash gauges to report the amount of water in the tanks. I'm not much help on how to fix, but just letting you know it's not just your boat. Jump over on
www.planetnautique.com if you haven't already done so. Those guys know everything you need to know to fix it, guaranteed...