Recently discovered a leak through my rear Roof AC. It had been leaking a little for some time, but went un-noticed because the vinyl ceiling "sponge-back" soaked up the moisture.
One clue was the vents closest to the inside AC vent had rusted screws from moisture migrating through the sponge-back vinyl ceiling. Another clue was moisture residue in the inside of the AC vent and wet sponge-back vinyl you can feel if you remove the vent.
Anyway, I thought I'd pass this on since it easy to check and might not be caught depending on how much moisture is penetrating the seal (i.e. could be leaking, but not visibly dripping).
In my case, it has been leaking a little for some time and got worse. My AC tie-down bolts were loose (these are visible if you remove the inside vent and look for 4 very long 3/8" drive bolts attached to the AC). This has been mentioned before, but in my case, simply tightening them did not work.
Tightening these up did not fully seal the AC as I couldn't get the roof AC seal to compress adequately (which can be viewed from the side of the AC on the roof).
I ended up adding an additional seal (opting for a white 2 piece seal, which was much easier to install than the black 1 piece seal). Since the original black seal adhered to the AC and was in good shape, I laid the new seal against the roof (it's adhesive backed) and "gooped" the ends together.
I was still able to cinch down (without over-torqueing) the AC onto its nylon stands and, at least in my case, surmised that the mounting of my rear AC would never seal properly with the original factory seal; it needed to be thicker.
Oh, almost forgot to mention... As has been reported before, once oyu're inside the AC vent covers, turn on the AC to see how much airflow is lost at your metal duct connections (just feel around the joints for airflow). I, like others, had a lot due to gaps between AC unit and chassis ducting. I just used duct tape to seal up the gaps and found I had much better resulting airflow.
2004 Alpine 34 FDDS
Anaheim Hills, CA