Hi,
It is possible that the first owner has not done calibration filling ever on the smartcraft system (fuel).
There may also be electrical corrosion somewhere water/fuel tank wires.
Here's a link to the fuel meter kalibration, read also your NT folder for smart craft tank calibration which is done in 4 stages, as I recall.
http://sentoa.org/maintenance_tips/f...alibration.pdf
I thought the water tank had a similar sensor as the link where a fuel tank could so help you?
NBs