Calibrated correctly I assume? Does it calibrate correctly with ignition on? Check connections and add ground wire. Check connections on the gauge. Pull it out and test it. Should read empty when out and check the reading as you lower it into a full tank, takes time due to the high damping. Run temporary wires from the gauge to the sensor.
Could be the gauge of course.
2009 Holiday Rambler 42' Scepter with ISL 400 Cummins
750 Watts Solar Morningstar MPPT 60 Controller
2014 Grand Cherokee Overland
Bob