Once you remove the back walls to the basement storage area, you should be able to see the furnace. If you have a drop frame, like the larger rigs do, then the furnace may be mounted on an extension of the main floor. Which means removing the underbelly covering will not get you access. In the photo, you can see how it's mounted in my rig in the center rear of the photo, next to the water heater. Not the most accessible thing to work on in place. It is possible, as found by others, that the flexible duct work was cut too long and has a crimp in it. I removed about 3 feet of excess from my bedroom duct that was hanging over the PEX plumbing. Yes, boys and girls, it's my old "Basement 'o' Doom" photo.