This is a pretty common thing with several other vehicles as well. I know in the Toyota truck world, there is a gent that now makes a plug-n-play harness that connects to the taillights and number plate lights; when it detects that the number plate lights are on, and then reverse is activated, it cuts the power to the number plate lights.
It's a fairly simple normally closed relay that is opened when reverse is activated. You could grab the signal for the reverse light activation from the trailer towing connector if you have it and that would keep everything "outside"
