SMIB, first jack up each wheel off the ground, adjust each wheel brake shoes for a good fit. (the installers just half a-- the stuff together).See if this improves your brakes. If not, like mine (it was in the shop four times for brake problems) you could have a bad electric magnet in one wheel. I could not lock up my brakes to test to see if my king pin was locked properly. You can also check when you are towing by feeling the brake drums to see if one is "COLD" (not working). That will give you an indication of each wheels brakes are working or not. My controller was like yours, highest setting. I even purchased a new controller (didn't change).
Each time it was in the shop, they would say fixed, brakes were working. Well, they were not fixed. My brakes will now throw you through the windshield. I'm towing a 31' fifth wheel. I can sit still, pull the manual lever over on the controller, I can not move the 5 ver, all four wheels are locked.
A shorted magnet in the right rear wheel was my problem.
This will give you a start to trouble shoot your problem.