Look behind the front streetside wheel. A low pressure puff will probably take care of it from the bottom with a minimum of fuss.
If your air conditioner has been replaced with a Carrier V, unless they did Inland Andy's drain modification, it's just the way they work. They blow the condensed water into the coils to improve efficiency. This may work SOMEWHERE, but in humid areas it makes drips down the sides, and mildew follows.
Since your '80 was made in '79, you can consider yourself a honorary 70's Excella owner, so welcome.
If you obtain the owner's manual, you can settle a mystery that comes up now and then. Rumor has it that the '80 owner's manual should describe the '79 Excella. I have the '79 manual that says it an Excella owner's manual, but it doesn't even show my trailer's front gaucho, mid-bath, rear twin configuration.
I don't imagine they changed TOO MUCH in between '79 and '80, so if there's anything I can help with, just holler.
Also, I've heard that Airstream has reprints of some of the Owner's Manuals. That would be worth checking out.