Brad,
This does sound like a short at the Trailer side of thew wiring. The easiest way to confirm this is to use a Multi Meter and check for a short to ground. As stated above remove the Light Bulbs and disconnect the Trailer from the Coach. If you have an older Analog Meter it will Peg the Needle when you find the Short to Ground.
While there are many suppliers I know of this Test Plug at West Marine;
http://www.westmarine.com/webapp/wcs...7&classNum=426
Good Luck I think that you are on the right track,