I just sealed off a leak to the rear air conditioner. Sometime in the past someone used silicone seal around the four castle nuts that hold on the fiberglass cover. Needless to say, water got in thru the nuts/flat washers and migrated down thru the insulation under the sheet metal cover, onto the insulating sleeve that covers the copper tube that goes to the control on the front of the air conditioner inside. Water then proceeded to drip onto the carpet in the hallway. Three days of rain but I caught it with a bucket until today when I resealed everything. My suggestion re this is to check the seals around the covers on your air conditioners just in case!
It sounds like you thoroughly checked the cause of the leak but my 93, U225 has never had any sealant around the cover and has never leaked and yes we do have rain in Oklahoma, 13" in 24 hours in June. I have had leaks in both front and rear A/C's due to old, worn out roof seals. I replaced them with cheapies once purchased from eBay and had to go back and replace them again this fall with the good ones from Foretravel. Just a thought.