Daniel, this is a strange fault. I will assume your wiring is similar to my 1988 Excella 25 foot. Behind the front couch is the 12 volt
distribution center, concealed behind a small panel. This 12v
distribution center is fed by the battery, the Univolt, and the charge line from the tow vehicle. One of the 12v
supply wires from this center travels down through the floor, and through the inside of the driver's side A-frame member to emerge near the front jack. Here it splits, and one live wire goes to the breakaway switch for trailer braking if the trailer becomes detached from the tow vehicle. The other branch of the live wire goes to the power jack. The ground returns are via the frame. To understand better what is going on, you will need a voltmeter applied between the live wire at the jack, and the frame. A couple of questions spring to mind:-
1. The jack works when the tow vehicle is connected, but not on shore power. Does it work when there is neither shore power nor tow vehicle? i.e, on the trailer battery?
2. Has it been like this since you have owned the trailer? If so, it is possible that the wiring has been altered from it's original layout. A wild guess would be that, as in my trailer, the concealed wire had broken in the A-frame. It is difficult to re-thread a new wire, so it would be easier to take power direct from the charge line from the tow vehicle, cutting this line from its connection to the 12 v distribution center.
As the 12v
distribution center should be simultaneously supplied by all 3 alternate sources (when present), it's difficult to see how an accidentally broken wire could cause the fault you describe. My knowledge on this is very basic, so let's hope someone more knowledgeable willl come up with some suggestions. It will be interesting to know the final answer. Good luck. Nick.