I think the Springdale TTs use a 30K furnace. The bottom of the plenum is open to the duct in the floor. There should be side knock-out plates on the plenum that rotate to come off. Using a flashlight and a mirror check for obstructions. Do this at the nearest vent also. (you will need a #2 square drive screw driver to remove the 2 screws) I have had good luck in checking all the floor and ceiling vents this way and sealing poor fitting joints with metal duct tape. (do not use the cloth duct tape as it will come loose and cause more problems. There should be at least one flex duct connected to the side of the plenum ... 2 or 3 inch duct. If this has come off, most of the air flow will be going into the cabinets. Hope this helps, Hank
You may be surprised at the amount of construction debris you find in those ducts.
Hank & Lynn
2007 Cougar 290RKS, E-Z Flex, 16" XPS RIBs ( SOLD
.. Gonna miss her ... looking for new 5r)
2004.5 Dodge 2500 QC, LB, 5.9HO, WestTach gauges, Ride-Rite