Either your door window or lock have a leak.
My window and door lock both leaked. I washed the trailer and noticed the door was wet on the inside. I pulled the interior trim ring on the window and sprayed the outside again, sure enough, saw a small drip where the factory seal to the glass leaked. Fixed that with silicone around the glass on the exterior.
The door lock has a foam gasket on the exterior. When the factory installed that, they folded a corner of the gasket causing another small leak to the inside. I pulled the lock and applied butyl tape over the gasket and reinstalled.
The doors are just filled with styrofoam and water gets inside the skin from leaks and runs down to the area you pointed out. Try both the above fixes.