Leaking on Galley Side
Hi, I had the same issue of leaking on the kitchen side of my trailer, thought it was the vents (plumbing stack cover, refer and stove and roof vents) so I replaced all of them, she still had water coming in. Then we replaced and sealed all the exterior running light fixtures. Still had water coming in the vent areas... Then I spent a lot of time sealing each rivet on the entire trailer one by one with Captain Tolley's Creeping Crack Cure ...repeatedly...which definitely did help a lot but I still had water coming in the galley side. It ended up being the overlapping seam above the windows that runs the length of the trailer. It wasn't till I got frustrated and went out there in a rain storm and pulled all the upper interior panels off that I found what was truly the problem. I found it only has about a one inch overlap between the big side panel and the roof panel and when the wind and rain played wrong together it was blowing water up and over the entire side in areas, showing up as leaks in what I had first thought was the vents and windows. It was definitely tear worthy all that time I spent doing things to her only to go out the next time it rained to find more moisture infiltration but on the plus side now that I have repaired that long seam by applying Aluminium tinted gutter seal to the length of her she's a happy trailer now for a long time to come as every possible thing has been done to her now :-)
also I know by sitting in my trailer watching known problem areas it is absolutely amazing how much water can come in through a minute hole via either a rivet that sits a little to high or an air bubble in your sealant of choice. Take the time to look it over very closely even a gap 2 mils long lets a lot of water in. Also water can travel quite a distance before finding itself an exit so the one thing you do know is your leak is at level or above where you are seeing it come in from. Check the condition of your rear running lights to on the back quarter panel, as water can come in there and if your layout is similar to mine it is on that height level so that could be your source as well. Another possibility is some plumbing stack cover styles have a weep hole on the lower side that needs to be kept clear as well so water can escape the cover area, someone else that has that style may be better able to advise on what that style of plumbing stack cover looks like so you know if you need to be looking for weep holes on yours and if your stacks have the rubber gaskets under them, Andy at Inland RV says those are only supposed to last about three years before needing replacement, so there is another possible infiltration site for you to look at.
Oh and one other thing make sure you have the correct rivets everywhere. If you have any of the pop style ones water will travel through the hole in the middle of them. Swap those out for Olympics (or buck) if you find any on your trailer.
Captain Tolleys Creeping crack cure. Sitkaflex 221 and Aluminium tinted gutter seal are all your friends right now