Pull the wheels on the trailer, check all the wiring in the hubs around the magnets, and then trace the condition all the way back to the pin box.
I would suspect that new trailer before I would the truck. I bought a brand new Montana and fought alarms on my Ford Integrated Controller all the way to Montana and back, and for another 6 months after. It came and went, but what an annoyance. I spent every free moment trying to blame the truck. It took the Montana Dealer about 6 seconds to figure it out while in for cabinet work on warranty. He asked if I was having any trouble with brakes as he was swinging a magnet with cut wires inside the hub.
Next time, I get my jack out and pull the wheels. I was just lazy and was looking for anything else that might be wrong.