I'd suggest that before you start removing marker light fixtures looking for a bad ground, try a couple of easy measurements at the truck's umbilical socket.
With the truck's lights on and a turn signal indicator operating, measure voltage at the different pins of the truck's connector, including the ground pin. I think what you'll find is that there is power on pins that should not be powered, notably, the truck's ground pin connector. This is how the incandescent marker bulbs on the trailer are being powered.
I sometimes forget that LED lights are not "bulbs" as in reference to incandescent bulbs. They are a diode, a device that lets electricity pass in one direction and not the other. The light that comes out of the diode is a byproduct of that current flow.
