Because most RV furnaces have a few optional openings that the builder can use, for ducting an underbelly 'basement' where the holding tanks are, or they can run an extended duct into a bedroom at the other end of the camper...these are examples....
The one you found might have gotten bumped during installation, or maybe it was just loose and fell out on its own...who knows.
Turn your furnace on and open the cabinet to see if hot air is being directed where it should not be...such as a cabinet wall or a couple of plastic water pipes.
Or maybe the warm air is heading into the bedroom thru a vent....you just have to look things over.