What unit do you have? If you remove part of the wall in the basement storage area you will see the flex ducts run up to a foil covered cardboard duct that runs to the rear of the unit. There are several collars cut into a piece of sheet metal for the flex ducts to connect to. Mine required some pieces of channel to be put across the aluminum framing adjacent to the duct to hold up the metal plate and then re-sealing to make the air run to the rear of the unit. The rest of the duct can only be accessed from the bottom by cutting access in the plastic board.
2018 Redwood 3901WB pulled by 2016 Chev 3500 HD DRW. Goodyear G114 tires, Moryde I.S., Kodiak Disc Brakes, Splendide W/D, Tire Minder TPMS, Sleep # King.