HOLDING TANK GAUGESIt 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.
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.