Is the spot soft or damp? The reason I ask, my old trailer had a spot like that above the door. It worried me to death until I took it apart. Come to find out, there wasn’t a leak at all. The paneling wasn’t cut to the right size and was just bubbled up.
As far as seeking out any active leaks, I think I would remove the bottom panel on the inside first, just below the transition strip. You can get a good look to see what type of damage really exists. You may have to remove/replace old insulation too. While the wall is open you can have someone flow the garden hose over the front of the trailer to locate the leak.
2009 GMC Sierra 1500 3.73 6-speed