The two other airs are what you might call "old school" ducted but not forward to back only side to side. The extra air I have is a non ducted model which will work well, its an old Carrier unit I replaced with an Atwood not long ago and I had the notion to use this old one in the center.
In a word picture it would go something like this:
Front air is an Atwood 15,000 BTU Heat Pump, ducted side to side.
Middle(to be installed) would be the non ducted Carrier 15,000 BTU heat pump.
Rear would be a Carrier 15,000 DTU ducted side to side.
IN my 96 era Vogue, the Ducted portion amounts to vents(large opening vents) on either side of the return air.
The side way mounting may come into play if I put this third unit where I have a Kool-a-matic Vent now. The issue is that my model of Vogue has a raised roof as well as a step up mid coach. There's plenty of space inside for the lower unit but the upper portion of the air would be right at or near the hump in the roof line.