Running lights cutting out on 05 M2 106 business class

hello everyone
I'm reaching out to you guys cuz I am stumped. 2005 freightliner m2 106 business class, c7 cat, Allison trans. The cheaper version of a sportchassis. I use the truck to pull a 40ft Eby gooseneck trailer..
Here is my situation....
Last week went to leave for a load early 1 morning. Fired up the truck. Ran idle for boit 15 min. Everything good. Started outta the door yard and down the drive way an i noticed my trailer lights to flicker once. Stayed on . Then shut off. I stopped. Got out. Check trailer plug. Notice I have no runnin lights at all. Truck or trailer. I had headlights, the side lights in the hood, brake lights an turn signals but no running lights.
So I parked it. Tried again on Thurs an fired it up. Lights all workin like they should. Started out again... lights cut out.
Unhooked it truck sat runnin for atleast half hour, lights stayed on. I Took truck to freightliner..they looked it over an said everythin check out. Claimed it was the trailer. Drove truck home today on the way home running lights quit again. No trailer on. Just bobtailed home.

Anyone have any ideas? Is it the headlight switch? Input? Thank you