Thanks for the post...wish I had seen it back in 2007. I was never happy with the braking on my 5th wheel. I had checked the output at the plug and was getting 12 amps so I chalked it up to the heavy weight of the 5'er.
I ended up buying a new truck that came with a digital controller installed. What a difference. The 5th wheel brakes operated exactly how I had expected them to. I had plenty of braking power.
I remember the power wire for the controller on the old truck was 14AWG plus it was connected to some sort of boost device (I bypassed it). Based on your post, I'm guessing an 8awg wire may have solved the poor brake performance.