I had the same type of brake problem with my TV and 5th wheel.
First the very best method of testing this is to use a DC clamp on type of Amp meter. Forget using the truck break pedal for now, and use the manual break lever for testing.
You should measure 4 Amp per wheel with the manual break lever at max. So that is a total of 12 Amp with all 4 breaks being used.
If you do not have 4 Amps going to any break with this method you have a problem with that wheels break. If you do not have 4 Amps the first place that I would check would be the where the break wire connects to the trailer wiring. This is normally some type of crimp on type connection and quality being what it is this is many times not well done. If this is a problem I would suggest soldering the wires together to ensure a good connection and covering the connection with heat shrink tubing.
Chuck & Sandra
2010 F350 CC 6.4
2015 Prime Time Sanibel 3601