Gunter,
Since one of the lights is working, I doubt you have a fuse problem since a fuse would normally protect both lights. Assuming your coach is wired similarly to mine, there is a circuit breaker in the electrical bay in front of the driver side wheel supplies power to both lights. It is OK since one of the lights is on. I suspect you have a bad connection at the driver's side light itself. Be sure the bulb is good. Ensure the socket is not corroded. Check for voltage at the socket. If no joy, consider using a pin to pierce the insulation to see if the wire has voltage.
Steve
