My unit is installed under the settee storage area in front of the gally sink with the return cut out in the same area facing the table. I was able to get ducts to all three cabins of the boat but this was a little tricky.
My unit is a 16000 btu unit and came with a 6″x6″x4″ fitting which i mounted right off the unit. So for the main cabin i ran 6″ flex duct through the port side settee it tucked in neatly along side the water tank into the vanity in the vee berth then up to the med cabinet. There is a hollow between the sink cab and the med cab. (look and measure before you drill or cut the holes) mount your adapted for your main cabin inside the med cab, off of that fitting i cut in a 4″ round fitting and ran across