Looks a bit like my old Lance 915... In that case, it was the frame of the cargo hatch leaking at the top (for the most part). Those door frames ore only held in place by 4 screws in the corners... If usure, I would take the door frame out; then you can clearly see if it ever leaked.
It could also come from anywhere further up - Lights, Window, really any hole in the shell. Water will run downhill and collect in the lowest spot :-(
It looks like you caught it fairly early and the damage is quite limited.
You can see that water was on top of the floor, as there is some slight water damage visible at the bottom ends of the furniture frame. The flooring covers the whole floor board and only has minimal holes in it for a few screws and staples; it is unlikely that water travels from below to the top in significant quantities.