Drain the tank and take a sample of the water. Is it clear? Regardless, I would try adding a good amount of tank treatment liquid and fill up the tank. Leave it full at least 24 hours. Drain and flush with COPIOUS amounts of fresh water.
Most tanks use a sensor attached to the OUTSIDE not inside, and it measures the resistance between air and water. If the inside walls get dirty/corroded up, it throws the sensor off and give a false reading.
John & Debbie
2008 Fleetwood Discovery 40X Full Timers Since Oct 2015