Electric brake effectiveness

I recently purchased a new bumper pull trailer with two 7K axles under it and load it to about 12,000lbs. I am pulling it with an equally new Ford truck with max trailer tow and brake controller. While the brakes on the trailer definitely do something it does not seem like nearly enough. To test the effectiveness of the brakes I made sure that the break-away battery was fully charged, pull the pin and then pulled the trailer with my 5,600lb truck on gravel. I then put the pin back in and repeated the experiment with the brake controller maxed out. I got about the same results both times: the truck struggled but did pull the trailer and the trailer brakes did not lock up. This tells me that the braking effort of the two 7K axles combined was less than 5,000lbs. That would explain why it takes a long time to stop. Since the brakes were made by Dexter I called them and they told me that it was normal for new trailer brakes to have as little as 25% effectiveness and that as many as 30 hard stops and an adjustment were needed to wear them in before they work at full potential. Has anyone else ever heard of a similar situation?

For some background info: I am getting 11.3V in the braking circuit at the trailer tongue which drops to 10.3V at the rear axle. Each axle draws 6amps. Dexter recommends 10.5V at the rear axle so I am not far off the mark. I am planning to improve the wiring by pulling a second wire to reduce voltage drop but that is not really the question I have. I am wondering about having to do an extensive break-in and brake adjustment before they work properly. The trailer company seems to be way out of their element on this sublject.