5.5 volts is the standard operating voltage for almost all native LED's. Those that work on higher voltages have to have a voltage dropping circuit ahead of the LED.
I don't really think that there were any RV antennas back as far as when your BigFoot
was built that were powered with that low a voltage. Did you read the voltage across the two orange wires? It that isn't "0" I'll bet it is just reading leakage from the circuit card. The good news, as the two leads going to the antenna should be insulated from each other at the antenna you won't hurt anything.
BTW: Did you find the other end of the wires to the antenna? Does it say something like Winegard anywhere on the plug plate?
Are you still sure you just don't want to sell it to us and ferget about this issue ??? (LOL)