David and Roy,
I think that both of your coaches use a pressure sensor in the side or end of your tank, near the bottom. As the level in your tank increases, the weight of the liquid in the tank adds pressure to the sensor and the electronics in the sensor system read the increased pressure as an increase in level. I think that the 2000 was the last Marquis that used the proximity sensors that Steve was describing.
Roy,
Your erratic readings are not caused by an air bubble, but it is either a restricted tank vent that is causing excess pressure in the tank as it fills, or a defective sensor.
David,
You can correct some of your sensor problems by removing the sensor, and installing a street elbow where the sensor screws into the tank with the open end facing up and screw the sensor into it facing down. This leaves a very small air pocket at the sensor diaphragm so that solids will not cake up on it. I would also install a new sensor while I was working on it. Your black tank sensor is in the end of your tank, just behind the stainless steel panel in your sewer bay.
Gerald