If you have a volt meter or test light, check all the terminals at the truck 7 pin connector and see if the problem is present there, too.
In the factory wiring, the brake and turn signal voltage are grabbed up under the dashboard somewhere, not at or near the lights themselves as in an aftermarket install. So what you see looking at the back of the truck may not be what is present or absent at the 7 pin connector.
Does your truck have separate brake and turn signal fixtures? There may be a problem in the magic box that converts 4 way to 3 way wiring.
In the trailer, there 3 single wires from the 7 pin to the lights, one for left stop/turn and one for right stop/turn and one for parking/running lights. If the turn signal works properly on the TT and the brake does not, then the stop voltage is not getting to the 7 pin connector.
Based on your report, the turn signal is performing normally? It is full strength and proper flash rate? If it is dimmer than it should be, then there may be a bad ground. But that is separate from the problem you're reporting.
'12 F250 4x4 Super Duty PowerStroke 6.7 diesel
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