trailer lights

[in2trux]

Instead of a grounding issue it sounds more like a hot short to ground in the turn signal circuit if the dash indicator is staying lit. Check the breaker to see if its overheating. Look for rubbed bare wires. It's doing the same thing with different trailers then it's on the tractor.
I would start with the tractor taillights, turn signals since they take such a beating from road debris and salt.