Sorry about that...should have been a little more detailed. You have plumbing stacks and the stacks have a rubber gasket and bug screen, they are all under the cover you see. One of my rubber gaskets was almost completely gone. When you install the new ones they are a very snug fit to the pipe and have sealant between them and the pipe. FWIW I also had a messed up screw in mine that was leaking....a factory defect from 1975...they had hit a rib putting the screw in and rather than drilling a proper pilot hole, they just glopped some more caulking on, it was leaking. The lifespan of Vulkem caulking is about 20 years, so if your unit is more than 20 years old I would seriously consider cleaning out all the old caulk you can find and resealing with the Vulkem and Parbond in the proper locations. It can be time consuming but I shouldn't have to do it but twice in my lifetime
Also forgot to mention, look for loose or missing rivets on the outside of the shell, loose rivets typically will have a small black ring around them caused by the movement and oxidation of the aluminum dust. I several that where just flat out missing. At least one I suspect had never been installed. The awning rail is another primary suspect because it runs the length of the trailer and water can enter under the caulking and run quite a ways before gaining entry, again you will need to remove the old caulking and reseal with Vulkem.