RJ is correct, but this is not only a GS problem.
On the 5th wheel we used to have when the installer cut the ductwork he just cut both sides and the top of the piece and bent it down, This restricted the air flow 50% or more. What I did was remove the inside AC cover and remove the cut pieces in the ductwork. I had to bend them up and down several times to get them to break off and remove them.
I also had to do this on our previous motorhome to the rear air conditioner ductwork.
Mine was first hand experience.
Ron