Here is how the back wall butts up to the floor. I circled the gap you may be referring to? The wall does not butt to the I-Beam. Critters can't get in and the foam you see, was for the electrical.
Your year unit, falls in the range, where the back wall needed to be fixed. You may be able to call Alliance and see if there is a record for your VIN# being repaired, if you bought it used. There is a angle piece on the inside, which screws the wall to the floor as well. You can open up your sofa and see if your screws are snapped or stripped. That is a good place to see if your back wall has been repaired or is moving.