Braking issue, need help troubleshooting.

"This just started happening after switching to a different trailer." So the problem is highly likely to be in the trailer. Do you own or lease the trailer? If not you call and tell them that the trailer is unsafe and needs to be looked at and/or repaired. If you have a spare cord I would swap that and see if it makes any difference.

 

I'm wondering why being 1200 miles from home is important.

Different trailer and brakes acting up.
Maybe water in the trailer brake lines?
Water does not compress but it can move around, perhaps causing erratic behavior.

 

Does your truck have a trailer hand brake?

 

Your anti lock is acting up.

 

Have you drained any condensation from your trailer air tank back above the rear axle?

 

The trailer is new. That means the brakes and drums are new.

The truck is old. The brakes aren't working. Out of adjustment. Warped worn out drums. Cam bushings.

The trailer doesn't require much pressure to make new brakes work. The truck requires more pressure to stop. The truck struggles. The trailer locks up. The trailer is doing most of the stopping.

I once drove a truck that was so bad. It didn't have much of a chance stopping without the trailer.

 

The FLD120 does it even have ABS ? Just guessing it doesn’t and the new trailer will have ABS. The new trailer might have disc brakes also. If you’re driving FLD with ABS or not working ABS. Then pulling a new trailer with ABS and maybe disc brakes could make for interesting stopping experience when empty. Like other said it sounds like the trailer ABS is activating.