Here's a possibility-
When I bought a used truck to tow my 5th wheel. I tested the controller output at the 7 pin plug. It was sending the correct voltage. For years I tried several things because I felt I wasn't getting enough braking from the 5th wheel. Adjust the brakes, check the output, clean electrical connections, inspect the brake internals and magnets...all were OK. So I learned to live with it.
A few years ago I bought a new truck that had been customized to tow heavy trailers. It had an electronic brake controller installed (DrawTite). The first time I towed the 5th wheel with the new truck, I couldn't believe how great the trailer brakes worked. The new controller made the brakes work the way I had always expected them to. I guess even though the old controller was putting out the correct voltage maybe it wasn't supplying enough AMPs.
I'm happy now...