P3 set to 13 volts? I may need to re-read my instructions but I thought it was a gain setting not a voltage. OK if set to max gain (13 volts?) you may not get lockup on dry level pavement but the brakes should pull you to a stop very fast. You should really feel it. What boost setting are you using?
If they seem weak I would first check the amp draw at each wheel. You should have close to 3 amps measured with your clamp-on DC ammeter while your buddy activates the manual lever. This is done while stopped.
Assuming that works you may need to pull the drums and inspect.
I also assume you have already correctly adjusted your brakes and run them in a few times if trailer has been sitting is storage.
Worst case start looking at disk brakes if the trailer continues to push the truck above 50 mph.