Start by playing water on the lower side of the trailer at the bottom of the window with a garden hose. Spray for 5 mins. or so. Wait for 20 mins or so to see if water appear at the floor level. If not move up the side of the trailer to the top of the window. Again if no water move up to the next area with a mechanical joint. Do this until you find the height of the leak or reach the top of the trailer. Keep in mind you will have to increase your waiting time after spraying as you go up the side to be sure any water from that spraying has had a chance to reach the floor. Once you find the heigth of the leak you just have to look closely at that area for the opening.
I had water in the side wall of the refrigerator, not the outer wall of the trailer, and it was coming from a clogged drain hole in the shower vent 5 ft to the rear. They are not always easy to find but generally run down as apposed to mine running horizontally.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles