Electrical problem with trailer system

I have a 2003 Dodge Ram 2500 quad cab with the 5.7 Hemi. It is a 4x4. It has the factory trailer wiring for a 7 round RV plug with a 4 flat tap. My problem is that when I hook up either of my trailers to the truck, the left turn signal on the trailer doesn't work. Read on, this gets good. The trucks lights all work, turn signals, hazards, side markers, running lights all work. I replaced the 7 round plug due to finding a loose prong on the plug, it was for the left turn signal/running light. This didn't fix it. However, when I plug my friends Tundra in to the trailers, all the lights work fine. Obviously this is in my truck, but I don't know if there are any other relays, flasher units, multi-function switches, etc that I am missing. I found and checked all the fuses and they are in good condition.

Thanks in advance for any help.

I sure hope someone answers you
I am having the same problem except both left and right turn/stop lights
I know it is the truck because there is no power on the connectors at the cable connection. I have changed out all the fuses labled trailer just for the fun of it and still no good.
Aint this fun!!!!

It turned out to be the Integrated Power Supply which is part of the Power Distribution Center. not a cheap fix and a stupid design if you ask me
There are two hard wired relays that control the stop/turn signals for a trailer.