If your water tanks aren't showing correct water levels, there are a few quick checks you can do to identify where the issue is stemming from. The vast majority of the time, the water that is going into the tanks and lack of maintenance is the cause of the issue.
Start by removing the water tank cables from the OdysseyLink, Trek or Tank Guage. This will give the system an empty reading.
Once you have the 0 point level registered, wet your finger and place it across all 4 pins on the Display plug-in point. This will give it a full reading.
In this manner, you can test if the display is reading the signals correctly.
The main causes of sensor failure are:
Bore and well water can have bacteria that will promote corrosion.
Bore water high in iron will cause rusting on stainless. This begins with the ferrous material in the water rusting which spreads to the stainless. It can be cleaned off as it's normally only surface rust but this can be difficult to do.
High mineral content (Hard) water will cause scaling and build up.
In grey water tanks you have the added issues of soap and fats building up as well.
Leaving tanks full or partly filled in storage for long periods can also have a detrimental effect on the sender arm in the tank as even food-grade stainless steel (as used in the screw points on the sender arm) will start to rust under the right conditions.
It is recommended by many of the water gauge manufacturers to use a pre-filter to remove impurities that can cause the premature failure of the senders and to empty the tanks when they are not in use.
There are a number of proprietary chemical products you can get that are designed to clean and flush out the tanks also. Normally you add these to the tank and fill it halfway before going for a drive to slosh it around before emptying and rinsing.