Winnebago uses the Precision Circuits "OnePlace" tank monitoring system on many different coaches. The same monitor panel is used for multiple and single tank installations. There is a set of terminals on the rear of the monitor screen used when there is more than one black or grey water tank. Apparently your monitor has the terminals connected even though you only have one grey and one black water tank.
Here's a link to the Precision Circuits manual.
http://www.precisioncircuitsinc.com/wp-content/uploads/2014/08/00-10032-300-One-Place-Monitor-Panel-Manual-A.pdfOn the bottom of the second page is a diagram showing the connections for multiple tanks.