[JRTJH]

I don't see your refrigerator drain line in the photo, so I'd suspect that it's either missing and the condensation is dripping onto the OSB floor, running to the edge and has destroyed the wood side under the vent frame and/or you also have a leak around the perimeter of the vent frame that is allowing rain to drip into the area and has rotted the wood.

If I were you, I'd remove the vent frame, pull the aluminum siding away and see if you can access the wood sheeting. You may be able to see all of the rotted wood and get access to it by working from the bottom of the trailer and through the vent cutout. If so, you can repair the side sheeting.

With that said, I'd also suspect, based on your photo, that the OSB floor under the refrigerator has also been wet and may be damaged. The screws holding the 120 VAC plug to the top of the vent opening are rusted, so, to me, that indicates that there has been water "coming down from above" which reinforces the suspicion that the vent tube is/was not connected or was damaged and leaking water onto the refrigerator floor and that side sheeting.

You're probably going to need to pull the refrigerator out to access the floor. Since you're going to be doing quite a bit in that area, I'd just pull the refrigerator and make sure everything around/under it is also in good condition.