Just solved the same problem
I just had the same problem with my Forest River 5th wheel. None of the fuses were blown, but my daughter informed me that the lights flickered before they went out. I then pulled the switch out of the wall and started pulling the wires up and sure enough, the wire nut holding the 4 wires together was completely burned up. After cutting them past the burn and giving them about 1/4 inch of wire, I checked the ohm reading to each of the lights it was powering and they all came out perfect, so I'm guessing the installer just didn't tighten the wire nut enough and it was arcing, eventually causing it to burn up. Works like a charm now.