If I was totally confident in the diagnosis by the electrician, (is he competent?) I would disconnect the old wire and run a new one. Then it doesn't matter where the old one is.
It's surprising to me that the wire got 'broken' in the middle. Usually it's a problem on one end or the other. Was there some sort of construction going on and the wire was cut? Might be a clue to where the trouble is. If the motorhome wasn't ever cut into the wiring should stay intact.
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