You have a leaking window. The reason that you do not see water tracks on the wallcovering is that it is a vinyl or plastic material. The water has seeped down between the wallcovering and the luan wallboard and the layers of the luan are beginning to delaminate. If you remove the inside window trim and look around with a good light, you'll probably see where the water came from. This needs to be addressed before the water makes it's way to the luan on the exterior. On the construction scale of strength and reliablity luan is at the bottom. RV builders use it because it's light and cheap, not because it's a good product and it gets really unhappy when it gets wet. The highly touted "Fibron" that the RV builders like to talk about is nothing more than luan plywood with some fiberglass chopped mat sprayed on it to hide the wood grain and a gelcoat covering. If water is allowed to get to the exterior sufaces the luan will delaminate pretty quickly.
The cure for all of this while not phyically difficult, is labor intensive. You can trust me on this because it was the exact situation in the marine industry with leaking ports. The only difference was that the cabin tops are fiberglass and if any wood did get damaged it was cosmetic and it could be easily refinished or replaced. Sorry for the side track..
Remove the interior moulding.The window will need to be removed and I'm not sure if it's set in with screws or if it's just held in by the sealant. An inspection will tell you. More than likely, the screws in the inside moulding screw into the widow frame and suck it in. If it is just "glued" in, you can loosen it with a sharp putty knife. Run it around the window going under the flange and gently lifting. You don't want to pry hard enough to bend or distort the window frame. It may help to have someone pushing from the inside while yu are prying on the exterior side. The window may be difficult to get out, but it will come out.
Once it's out, all of the old sealant must be removed from the window and the wall around it. General cleaning, wiping, is not good enough. It needs to be chemical squeaky clean so the new sealant will adhere to all of the surfaces. Acetone works well, as does MEK. Home Depot has both. The main thing is just get it clean. Use latex gloves when working with this stuff.
I don't know what the manufacturers recommend for sealant. use what they recommend. We use silicon in the marine trade, but that may not be the sealant of choice. Apply the sealant around the window frame. Don't go cheap here. Use plenty of it. When you place the window back in place and press it in, you want to see sealant ooz out around the frame. Anywhere that it doesn't is a potential place for a leak. Put the interior trim in place and fasten it with the screws you took out. I'm guessing the screws will go into the window frame and draw it up tight. Once the window is in place, you can go outside and wipe off the excess sealant with rags and acetone, alcohol, whatever. if you are having one window leaking, the rest are probably not far behind. It's best to make just one big mess and be done with it.
I would even go one step further and contact the manufacturer of the sealant you use and see what the life expectancy of it is. It doesn't last forever.
I know this is rather lengthy, and I apologize, but this is the only way to head off fiberglass delamination and damage from moisture. It sounds bad reading it, but over the years, I have resealed dozens of ports using this procedure and it's very effective. You can probably do your entire trailer in a weekend, maybe two depending on the number of windows you have.
I hope this saves someone from an expensive fiberglass repair bill..
I don't do fiberglass repair, but I have colleagues that do, and it ain't cheap.
One word of caution.. Once you start to reapply the caulk or sealant, you'll need to work quickly. If you are using silicon or a polysulfide or polyuerathane product, it will start to skin over in 20-25 minutes and at that point, any sealant added may not bond with what's already been applied.
Good Luck..
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