Hehe You should see the way ours was wired. Had a current umbilical that is spliced into the the last 3 ft of the original umbilical just before it went into the body. At that junction was a peice of armorred cable running up to that plug for the 12v
I'm replacing the whole umbilical again with the 12v feed from the umbilical making it's connection on the inside.
I do plan to retain that plug for a few reasons. I'll make a jumper so that if the tow rig is nosed up to the coach I can lift the hood and charge the battery. I can also use that jumper to charge off a Generator. You could also use that juntion to pull 12v from the coach to run something else. Maybe put a cig plug end on it to run 12v stuff like fan or a small compressor to fill kids blow up stuff.
Now where to find one: Hubbel(sp?) is a company that makes a lot of simular plugs. I bet they have one that size as well.