Adding third roof air conditioner, fitment question.

Currently our 03 Beaver Patriot has 2 roof air conditioners, both are the Dometic Penguin 13,500's and are ducted into the center duct on the coach. There are (6) 6" X 6" ducts running front to back. There is a roof vent near the center of the coach that I would like to add another a/c in. The potential problem is that there is only 2 3/4" between the passengers side of the vent opening and the hallway wall. The existing duct work is approx 5" away from the drivers side of the vent opening. I am trying to learn if there is a a way to make the third a/c work either by adding it to the existing duct work or use it non-vented. My concern is that I would have to add more vents to the duct plus add duct to attach the new a/c to the existing duct work. Since I only plan on using the 3rd unit when in high temp environments, I think it would be easier to install a duct less model but I have only the 2 3/4" space to fit the inside plastic air return. I called Dometic who told me to call a dealer who said that they were booked for 2 weeks. Anybody got a solution? I have approx 19" width to work within the hallway.