Carol,
Sounds like you'll be fine. If it's one of those screws, it's unlikely it was being used to hold up anything under the floor. Maybe just a screw holding the floor down!
Makes you wonder how it could undo itself, when it can't be wriggled out with a pair of vice-grips! Probably something to do with the motion harmonics, due to the material physics of a fifth-wheel, coupled with the various road surfaces encountered at varying times and varying speeds over a given timeline period.
Or not.
Steve