When you pulled out the fuse that would have eliminated a incorrect wiring from fuse 12 to relay slot 5 and resulted in no power to the bulbs. Flip the RV tow port lid open and see if something, debris, water or dead bugs, is touching the blades. Here's another thought, remove fuse F34. This isolates the trailer side from the chassis side. If the lights are still on then pull fuse F33. Your shrinking your search area to before or after the central junction box. I hope it doesn't come down to isolating batteries.
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