Need help with furnace on Forest River 5W 346ROT6

I am trying to help a camphost at 10000 ft cold with his furnace. He had heat to all registers before we replaced the furnace with the same model SUB furance. Furnace heat up great and blows air very hard in bed and bathroom. We have no air flow to his kit and Living room. Close Bed and Bath registers and get no flow to Kit and LR. Unhooked duck only at furnace. Duck comes out of furnace then to duck in bed and bath then goes towards Kit across floor to wall and down wall to the Kit and LR. Pulled of registers and in LR looking towards wall nothing blocking way. In bedrm looked down duck towards wall and see nothing blocking duck.

I have no idea what this could be.

Thank you for any ideas to fix this.

We are in CO in the wilderness and 4 1/2 hours to nearest RV dealer.

Thank you
Tom