At max output you should have 12v. Check with a colt meter at the plug with someone gradually increasing the gain.
If that checks out check the adjustment of your trailer brake shoes. If the adjusters ( which are like most any other drum brake setup) are hard to turn or frozen , remove the drums. This will give you access to inspect the brake components, shoes, magnets, & the surface of the drum that the magnet rides on. It must be as flat & groove free as possible for maximum contact. Shoe surface is the same. Also make sure it's clean & dry in there. Good luck.