With a good meter I would check the voltage readings in the coach with the breaker on and with it off and see just what is happening. If the voltage is really dropping that much then you have some problem with the connection. Could be a bad extension cord. Could be a loose connection somewhere in that circuit. Or something else that would cause the voltage to drop due to increased current.
If you are near the 15 amp limit you could be dropping 6-10 volts thru the line depending on wire sizes and tightness of connections.