Was not NL, but Towed trailer and hauled camper with 2009 GMC 3500 and 2016 GM 3500. Both had factory trailer towing package with integrated trailer brake and fifthwheel/ camper pigtail in box. Alternated between towing trailer & hauling camper all the time. Never had any issues..maybe you have a damaged pin or loose wire in one of the plugs causing an intermittent momentary interuption enough to cause the DIC warning..... I would start at the truck end with GM wiring diagram, then Bargman wiring diagram, then NL wiring diagram to be sure all is connected compatibly,