NO DC Voltage unless camper is plugged in 120 volts!

I found the problem, chased 12 v thru the 55 amp behind the
steps. Chased it thru a large (2 wire) blue plug.
When I pushed back the chafe wrap I could see copper wire.
I dont think the lug was ever locked in the blue plug.

I may go back and shorting the other cable as it keeping
the short cable from seating.

Thanks all for the input! tenn.vol.