The ECU is going to use that sensor to determine how fast the output side of the transmission is turning in order to understand how and when to shift the transmission. The sensor itself should be located on the top of the transmission towards the tail, and shouldn't be a terribly expensive thing to replace. But before you do so, understand that the computer isn't telling you that the sensor is faulty - only that the data coming from the sensor isn't what it is expecting. There could be something else going on with the electrical system that is generating noise and throwing things off or there could be a wire that has rubbed up against something and become compromised. If the transmission appears to be shifting the way it normally shifts, then it could be a transient problem. If, however, it hangs a bit before a shift, shifts too early, or shifts particularly hard, then whatever the fault is should be a little bit easier to find since there's something actually broken that can be tested for. You'll know that things are really in need of investigation when the "overdrive off" light starts blinking.
Since you say that the coach is driving okay, I would go ahead and clear the codes with the scanner. If there's really a problem, the code will come back and further troubleshooting can continue unhindered. If the light comes back on, then the sensor can be tested to see if it's at fault or if there's some other electrical gremlin. On some installations, Ford ran the harness from the transmission to a connector to the main ECU harness and that connection point can introduce intermittent issues (rare, but possible). I've seen alternators spitting out bad power cause sensors to act a little weird, and I think there's even a TSB out there that involves the coil-on-plug system. And, of course, it's entirely possible that you've failed that sensor itself.