Many times the place where the flex hoses that come from the furnace plenum are connected onto a piece of metal that is taped to the bottom of the duct that runs in the floor. The tape lets loose leavening you with no air on that side of the coach. I would inspect that connection first and use straps to make the plate stay in place. Using your phone take pictures of the inside of the duct from above to see if it has collapsed or broken. Use eternabond roof tape to repair any holes holes.
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.