Electrical lighting circuit not working

I went to Motorhome this weekend to check it out. I found one of my 12 volt ceiling light circuits would not turn on, all others worked. I removed the switch and disconnected the two wires. I checked continuity across the switch and found it to be open/closed as one would expect as the rocked was moved back and forth. I measured voltage across the two disconnected wires and found 3.3 volts.

I believe this means I have something wrong on the supply line to the switch. I also wonder if it could be a ground problem. All wiring is embedded in the walls except for switches and lights so I do not know where the various wire connectos or ground points are.

Any ideas would be appreciated.