Just above the exterior of the side door frame ( door frame header ), you will see that there is a bead of caulking. The water is tracking down the back side, of the side jambs, of the door. I have see this in at least three friends trailers. To prove this is the leak, have one person in the trailer with the door closed. The second person outside of the trailer will turn on a garden hose and start at the bottom of the door frame, wetting the area ( do both sides of side jambs ). Work your way up both sides slowly, then do the top last. You will most likely find the leak is at the top, where the caulk has failed. The fix is to remove the old caulk and replace.
TV= 2012 F-150 FX4 Supercab
FW= 2011 270ret Crusader
Hitch= Reese Pro Series 16K Hitch