What I would do is remove the return air grid that covers the opening for your furnace and take a look at the ducts that connect to your furnace to see if one of them has come loose from the furnace.
I had this happen to our 2003 GS coach we used to have. It had a sheetmetal type connector that locked to the furnace by turning 1/4 turn. Evidently it had vibrated and came loose allow the duct to move away.
I reconnected it and all was warm once again. But it was not easy to reconnect since of the very tight limited access and my big ring size 15 hands.
This might be your problem, especially if it worked okay before and now it doesn't. Or maybe it was never connected when it was built?