You may have a bad wire going to the sensor, or the sensor is in the process of crapping out. Does the coach have ABS? I am guessing it has an Allison transmission. On some coaches of that year, there is a two-prong plug under the hood that goes to the speedometer, it may have corroded or gotten loose over time. Please let us have some more info, we can try to send you in a direction (hopefully the right one).
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.