I would have thought a wire tapped into the gearbox-mounted switch to a dash-mounted switch would be simplest.
Two problems maybe -
1. It would also switch off the reversing lights (which probably wouldn't be a concern as they only light up the caravan and could be better not on anyway) and
2. The danger of forgetting to switch it back on. Can't think of a circuit to light a warning lamp when power's off, but I'm sure it's possible
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