Go to your vehicle's fuse box, which is often found inside the camping cabin. Locate the fuse for the water-tank gauge and make sure that it is not blown or missing. If it is blown or missing, replace it with the proper type of fuse (consult the user guide or look at the fixture for the recommended voltage).
Unscrew the panel that holds the physical gauge in the wall of the vehicle. (The gauge is often located near the front door.) Pull the panel off and inspect each wire leading to the gauge. Check for loose connections, ripped wires or broken wires. Replace ripped or compromised wires and tighten all connections to the gauge.
Go to the vehicle holding-tank housing. (The holding-tank housing is most often located on the right-side exterior of the rig.) Open the panel door and find the black water tank. Because the tank holds raw sewage, wear eye goggles, a mask and rubber gloves to prevent hazardous material from getting on you.
Test the hose connections on the tank. Pull them to ensure that they are tight and not plugged or compromised. Unscrew the hose clamps and pull the hose away from the tank; check it for blockage or obstruction. Look above the hose -- near where it comes in through the vehicle -- and make sure that the wire is connected. If the wire is compromised, replace it.
