Does the fuse blow with the trailer plug disconnected? If it does, the problem is in the truck's circuit. If it doesn't, the problem is in the trailer. Assuming the problem is with the trailer, and also assuming that this wiring once worked with no difficulties, start with the trailer plug.

Pull it apart and inspect all the wires. Make sure there are no strands of wiring touching each other, and that the end of each wire is fully contained in the socket and held in place with the set screw. Look for broken wires. Your truck's fuse is being blown, by your running light wire touching a ground source.

Find out which wire is your running light source and pay particular attention to its routing through out the trailer. Check each of the running lights for integrity, again broken or chafed wires or a bad socket. All you need is one strand of one wire, touching a grounded spot to cause your difficulties. 

Using a battery charger and a nail inserted into the trailer plug sockets, is an easier and cheaper way to trouble shoot than constantly replacing fuses.

Gard

You're welcome. Hope they find your problem; it sounds as if there is a potential for many.

Best of luck    Gard