I have a 2005-2006 American Manufactures trailer 18' dual axle single axle brake setup.. Ok I started having issues with the trailer and wasnt sure if it was the controller or the trailer going bad, found out it wasnt my controller after towing a friends gooseneck.. Anyways I decided to pull the hub and drum off to inspect the system, what I found was a chaffed wire going to the magnet, so I cleaned it up and used electrical tape to patch the wire and move the wires so it doesnt happen any more.. Great no more faulty reading after I fixed the issue, I drove the trailer around empty and no issues until I loaded my buggy up.. Once I put the buggy on the trailer I lost all readings on the brake controller like the trailer wasnt even hooked up, but when I pulled the buggy off it worked fine.. All the wiring is good and I cant figure out what the hell the deal is with it..

I was having a similar issue, it turned out I was pinching a wire between the boards and frame.

the wiring isnt binding or in between the wood/frame, it routes through washers that are welded to the frame

Seen this a few times. You probaly have a broken wire inside the wire insulation(sp) on your trailer. Or in the loom that runs to your trucks connector. Get V/ohm meter take a reading on EACH lead with the trailer empty and then loaded. Should be easy to find this way. I know it sounds NUTS but that is how I found the last one.
Double check your ground from your truck to the trailer too
Respectfully,
LG

the wiring isnt binding or in between the wood/frame, it routes through washers that are welded to the frame

I'd also double check the wire at every washer, may have rubbed through.