Fresh Water Level Indicator Not Working

Yes, have to agree you did look 'reasonably' clean  and yes, why do we use so much water 

Does this suggest that the cause of the furring up is a very small but continuous flow of electricity through the studs?   

Maybe Sargent could be persuaded to change the method of sensing so that there is no electrical flow until the user wants to know the water level and then simply presses a button...

"Hard water is water that contains a higher concentration of dissolved minerals like calcium and magnesium. When the water evaporates, it leaves behind calcium carbonate deposits. These limescale deposits therefore build up on any surfaces that hard water is in contact with and can accumulate quickly."

The furring is not caused by an electrical current.

https://www.autosleeper-ownersforum.com/t15622-sargent-fresh-water-level-indicator-studs?highlight=indicator

Has anyone tried one of these?

http://mcsboatproducts.co.uk/portfolio/fresh-water-gauge/

The basic principle seems much sounder

I have just had problems with both fresh and waste  tank indicators and on advice from my dealer I tried to cure by descaling. It did not work but after reading this post and from that understanding how the system works I have cured the problem.
      I was getting false positive readings ie showing more water in tank , not false negative readings ie less water.
Descaling can cure false negative readings by cleaning the sensor studs but to cure false positive readings I simply sprayed the area where studs are mounted outside the tank with WD40.
  The system is very sensitive and works on very little electrical flow. This means if the area around the sensor studs is not spotless then enough current will leak between them giving false readings, especially if vehicle is driven on salt gritted roads.

Hope this helps others .