Since you have found no water running down inside the trailer walls, could the bottom rubrail right next to the door where it sits next to the door frame be pooling water behind from the trailer length and wicking into the floor at the floor level? Picture below, talking about the rubrail to the left of the door. We had similar issues when we gutted ours. The plywood at each side of the door, against the frame had rotted a 2" X 2" hole. It was our door seals from what I could find. After sealing those, it dried up.