Question about converter

OK here is what I found out. The first thing I did was to sand the area and terminal on the battery chassis ground, no change. Second I could not get to ground from the panel, so it was easier to run a new one, no change. Third ran a jumper cable from pos. term. to panel, (bright lights 12 volts). Turned all the lights on, (15 of them) only dropped from 12 volt to 11 volt. I can live with that, that is a lot of lights to power without a drop. So problem lies between the panel and the battery. The pos. line runs into the trailer into a cavity, where the radio is in. Can not get the radio all the way out, but far enough to see a mess of wires (battery positive, light switch, slide out, speaker, coax, ect.) and they screwed a clothes hanging bracket up through it. Plus my battery disconnect switch is there. So the next step is to cut a hole in the top of the cabinet and look at the wires. Do the battery disconnect switches go bad? That is a connection that is buried in that space.