Damp at back of Duetto

Hi all, looking for help and advice please.Back of 1998 Duetto below driver's side rear barn door there is a damp patch. It is not showing leakage inside. seals are good, it is not leaking from fridge vents or rear lights- all dry. Any ideas please, could water be coming through the tapping /gutter rail? I can't see any obvious signs after clearing them. 

Thanks in advance

In my experience, leaks around the gutter are very rare. I would be looking at the roof vent as a possible suspect, but it could be coming from several other places, depending on whether it's parked on a slope.

You may be able to trace back to the source by strategically placing pieces of newspaper, as that shows up well when it gets wet?