I had something similar happen to me when I bought a used snowmobile trailer, my old trailer I would always be around 3 loaded. My new trailer was twice the size and would require a much higher number like yours, but what I found out after awhile was that I only had one brake working out of four. At first I thought I had a bad brake or bearing because one was always smoking hot and the rest were cool to the touch, turns the hot one was the working one and the other three where trashed. Fixed all brakes and controller now back to 2 to 3 1/2 depending on load. Good luck