Is there a fresh water drain on the bottom of the tank? You might try opening that to see if any fresh water comes out.
If you get water out of that drain, then your tank is filled and the sensors are bad. The tank should have an overflow on the top of the tank where water will come out when the tank is full.
Otherwise, there might be a valve on the top, but maybe someone with your rig will know.
2015 Dynamax REV 24TB class C