water tank overflows?

My a/sleeper clubman has a wet floor just where the cab joins the living part. This appears to be caused by water coming UP from the fresh water tank underneath, defeating the laws of gravity! I've ruled out it being caused by spray while driving as it still gets wet on dry roads. When I fill tank it overflows down the sides as others have described but how on earth can it come UP through the wooden floor and soak the carpet?

John
August 2014

they have a hole in the very top so if you keep filling it it pours out

Caratech
July 2012