Welcome to the forum. The furnace issue can be something as simple as a duct not connected to the furnace itself, do you get good airflow from the furnace itself? On my furnace, I have good airflow at the furnace output but very low output from the duct that is attached to it, I looked at the furnace itself and found the duct takeoff on the side of the furnace (it has a twist lock connection) was in place however the twist lock metal connection to the furnace was loose, I tightened it but it needs some duct tape to seal that connection on the furnace sidewall, I haven't done this yet but I need to see just how hot that connection gets before I stick any duct tape on it, a better solution might be to drill a small hole on the flange after tightening the connection and install a small sheet metal screw, you might try to locate the furnace and see what kind of access there might be to get to it and check that the metal twist lock connection is in place, don't know if this helps you.
2005 Springdale 249BH FW
2008 Toyota Tundra Crewmax 5.7L