Heavy rains last week demonstrated some issues, and it looks like most are seams, as they all stopped when I moved the bus uphill - all but one. The one dripping on to the driver's dash. The primary suspect is this guy. Not sure what it is or purpose, or even what problems I will cause if I caulk it/remove it?
I had one of those. Supposedly it is to allow air in / out for expansion and contraction of the air when the bus is in the sun and for day / night or whatever. Unless your bus is super airtight I don't think it is necessary. I took mine out and patched over it. No issues yet. Mine had that nasty mold under it on the panel not necessarily from leakage but probably just years of air / condensation coming through.
To find leaks, I simply went out in the bus when I had the interior totally stripped (ceiling panels out too) during a heavy rainstorm and made sure I had some bright halogen lights on inside. I had probably 6 or 7 different leak spots on mine, several from the roof hatches which I replaced with units from another member here and resealed. Some were leaky rivets which I just gobbed silicone over. Others were seams at the sheet metal. I used some gooey rubber roof caulk from Home depot to seal ALL seams on the roof (if you PM me to remind me I will link it to you). Finally, I used elastomeric roof paint to paint the roof white (heat reflection) and used a 4" brush over the riveted areas on the roof, and just slathered the pain on there real thick. All the other areas I just rolled it. That took care of all but one of my leaks which came from a side panel gap in an area where a lot of water was running down the side of the bus. That one I just undid / broke off the screws (Thomas built bus which uses screws instead of rivets in many places) and riveted it into place with some automotive seam sealer.
so I simulated a repair, and a rainstorm yesterday, with no leaks. Maybe I will leave the repair and wait for the next rainstorm? I hesitate only because I fear results of removing the vent. Would that not cause MORE moisture?