I ran into a similar issue on this past trip, long story but one will not tighten but turns. I plan to use a stud removed (I have a Snap-On version) to grab the end of the bolt and tighten the nut on the stairs while holding the bolt by the end. Have't tried it yet, but that is my current plan. I don't want to rip up any flooring if possible.
2005 Newmar Kountry Star 3742 - Gas
2018 Chevrolet Colorado Toad
Roadmaster Tow Setup