Fresh Water Level Indicator Faulty

The fresh water level indicator on our 2008 Sigma EK always shows 'Low' even when I know the tank has just been filled to overflow.
Before I start to dismantle anything, has anyone got ideas/suggestions/advice?

  Thanks

Lots of previous posts on this subject, this is one of them...

https://www.autosleeper-ownersforum.com/t27903-water-tank-level-gauge-success?highlight=Tank+level

Other suggestions are citric acid to clean the sensors or even to get a Henderson Hatch put in the tank so you can get your hand to physically clean them.

Thank you.
 I have a feeling that I do have a hatch fitted to the tank. I will have a look tomorrow. 
   Mungo

I see that you have the gear to go inside!  Take the shrink pills first!

Managed to get my arm in through the hatch (Didn't need my aqualung). Wiped the sensors.
Flushed with cleaner and rinsed. Seems to be giving the correct info. now.
  Thanks for the advice.

 Any ideas about this :-     I found five 'sensors' on the outside of the tank, each had a different coloured wire to it. A sixth wire(Black) hanging loose .   Is this  because they used a six core cable and only needed five? Or have I got a loose connection ?
   Mungo

As you can see there are just the 5 wires coming out of the loom.  If your van has a Truma water fill (and I would doubt it has) there is a float switch in the tank that disconnects the 12v from the truma fill point when the tank is full.  If the switch was installed and had developed a fault, then someone may have bypassed it and left the wire hanging

Thanks
 No truma fill and no float switch, I think it's just a spare wire.
    Mungo