It seems that most everyone will have problems with their holding tank gauges at one time or another.   Below is some input from Jim Pourroy of Fleetwood on this, along with some info for changing to a more reliable system.

To start with, a clean tank is a must.   Most of you know that and have tried all of the tricks and all cleaners.   The two not so often tried and very successful cleaning methods that I can add would be the hot water, and the opposite, the ice cube treatment.  

At the service center we clean the hard to clean tanks by connecting a hose to a large water heater, 40 gallons or so and fill the tank, either gray or black, and then drive around to slosh the water against the probes to loosen and clean debris.   The opposite procedure is to add 20 pounds or so of ice cubes (through the toilet) to 5 to 10 gallons of water and repeat the driving around.   When it melts it will drain out like normal and hopefully take a bunch of debris with it.  

Discovery's monitor panel works a little differently than other ones used in Fleetwood motor homes.   It uses a external resistor pod that sets up a voltage divider string.   This type of monitor is called the uni-wire monitor as there is only one wire for the sensor and one wire from the empty probe to ground.   This system uses a external resistor pod to connect the sense wire to the probes.   They can be tested with an Ohmmeter.

The best measurements will be obtained by removing the wires from the tank probes.   The resistors (two) are 68 K Ohm and are in series.   They should read continuity from the input wire to full probe green wire, 68K Ohm to the 2/3 probe yellow wire, and 136K Ohm to the 1/3 probe blue wire.   If the resistor pod does not measure as described ( plus or minus 10%) it will need to be replaced.   The part number is 112950.

The Discovery monitor panels also have potentiometers to adjust the sensitivity readings of the gray and black tank.  

1996 - 1998 Discovery monitor panel the potentiometers can be accessed on the side of the PC board.

1999 - 2001 Discovery monitor panel the potentiometers are accessible through the back side of the PC board.

Note the location of the pointer that looks like an arrow head prior to adjusting, this way you can return it back to where it was if needed.   It takes trial and error and a little bit of patients, only turn the potentiometer a little at a time.   Fill and drain the tank and observe the results.   The goal is to get it to read correctly with out being too sensitive, or not sensitive enough.   Depending on the local water and the tank contents there can be a fine line between the two.

Excerpt from a troubleshooting guide...

"Oversensitive readings can occur as a result of scum buildup on the tank walls, abnormally high mineral content in the water, or incorrectly located holding tank probes.  In these situations, the monitoring system indicates higher levels than are actually present in the holding tank.  Certain cleaning products and food by-products can build up on the inside walls of the holding tanks producing a layer of scum that can cause the monitoring system to read higher than the actual level".  

If cleaning and adjusting doesn't work then we add new probes.   The replacement probes are called well nuts work like expanding freeze plugs. We drill a 3/8" hole and then install them with a stainless steel screw which causes them to expand and seal against the tank.   The part number is 002788.   The screws make the probe serviceable.

If the condition persists, move the common ground probe 7 - 10 inches away from the other tank probes.  The increased distance between the ground probe and the other probes will decrease the sensitivity of the monitoring system.

I hope all of this helps more than it confuses the issue.   This should address the most common problem, false readings, but like anything there is even more to it than I mentioned.   You could have a defective monitor panel. Playing the odds though, the problem is almost always in the tank.
-Jim Pourroy

As an alternative solution, some D owners have installed a new monitoring system that measures capacitance to determine the holding tank levels.   One such product is the Acu-Gage, available at Camping World.   At least one member recommends getting the model with the LED display as it is $56 cheaper than the one with the digital display, which really is no more accurate.

Installation on D's with the older-style display panel by the refrigerator is not hard (up to the 2000 model).   However, several wires must be connected between the new display panel and the sensors on the holding tanks.   On later D models, you may want to locate the new display in an area where you can easily run these wires to.