A couple of things to think about.

I just bought a Jayco 5th wheel weighing over 15,0000, loaded.and the trailer brakes wouldn't help stop the trailer. The dealer said the trailer brakes were normal and the problem was with my truck.

Disconnect the trailer's electrical cord from the truck and pull the trailer break away switch cable. With my clamp on amp meter connected to one of the wires going to the switch, I was getting 13 amps. I put the truck in drive and the truck started moving with a little bit of throttle. (Leads me to believe the trailer brakes).
I then put the brake away back in the trailer. Plugged the trailer back in the truck., set the gain to 10. Behind the 7 pin connect I put my clamp on amp meter on the blue brake wire and was getting just under 13 amps. Once again, put my truck in drive and with a little throttle, the truck moved. (Dealer still said nothing wrong with the trailer brakes.).

When i towed it back to my house. I pulled out my IR temper gun. I away take the temperature on the back of the brake drum through the rim at the 9 o'clock mark where the longest brake pad comes in contact with the drum. 3 drum measured under 100 degrees and one was 136 degrees.

Dexter states after 200 mile of installing new brakes, they should be manually adjusted even if they are the self adjusting type.

One of the trailer brakes needed 30 clicks of adjustment and the one that read 136 degrees, i backed off 10 clicks.

When I was done adjusting,I pulled the break away cable and put the truck in drive, with a lot of throttle needed to move the trailer.

I then went on a 10 mile highway ride and the TBC gain set at 8, I believe the trailer brakes were working. I took the temperatures again and the tire tread temperatures were 136 degrees and all brake drums at 9 o'clock 100 degrees take or give a few degrees.

Sorry about length, hope it helps someone.