Here is the simplest way to check the running lights on the trailer to help pinpoint whether the trailer is the problem or the truck (tow vehicle/TV) is the problem...
Short pins 3 and 4 on the trailer 7 way plug. If the running lights on the trailer light up, then you have a problem either making a good connection at the 7 way plug or the TV has fuse blown or wiring problem.
Read the following article on pins 3 and 4 of the trailer 7 way plug
http://rvbasics.com/techtips/emergen...er-lights.html