Many of the newer controllers only give you an accurate voltage reading with the right load installed. Most RV shops have a special tester that puts the proper resistance on the truck connector so it can measure current/voltage properly.What should the voltage reading be at the truck's plug? I suddenly do not have full braking on my 5th wheel. When checking the plug with my voltage meter it bounces between 2.88 v to 5.58 v. both with the controller set on 6.0 and when manually moved to 100% on. should it read 12 volts?
I would bet your controller is fine but you have a corroded ground wire or bad splice near the brakes on the trailer.