That seems like a very narrow use case of the 12V to a trailer. If you are in the truck with it running, why would you need the interior lights to be on inside the trailer? Someone at Ford did not think it through, as interior lights on a trailer are fairly common and you need them on when you are NOT in the truck and when it is NOT running!
WTF?.....Trucks for years have had their 12v charge system tied into the truck running!...the 12v system is NOT to run interior lights, AND it never was!
It's there to charge the on board battery that modern trailers have during towing...that's it...
Install a small battery..and light up your life