I am not sure, because I have not tried it but here is a diagram of the wire connections for a 5th wheel trailer and more..
http://www.etrailer.com/faq_wiring.aspx
If I am correct the terminals are 3 & 4 and are on the opposite side of the notch and terminal 1 & 2 are on either side of the notch and that is the trailer breaks and ground.
Be sure what terminals you are shorting when you the terminals on your trailer. I believe the 12v is terminal 4 and the trailer lights are 3. But these are on the opposite side of the notch if the drawings on the web pages I find are correct.
FWIW