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There's lots of imformation on the "fixed it club" pages which should help you fix it.

As to what caused it, I believe it to be galvanic corrosion.

I have a similar problem in my 1988 Lunar Ariane but only on a 6-8inch run along one wall.  This area was damp and when I stripped it I found that behind the holes was a wooden batten that the plywood interior was stapled to from the inside.  The staples used on this section alone were slightly longer than the rest. They had come all the way through the batten and were touching the aluminium wall of the caravan.  Over time the aluminium will corrode instead of the steel, especially if there's a bit of damp in there.  I found that my awning rails were leaking which caused the damp. 

As I'm doing damp repairs, I've replaced the batten, gluing it to the Aluminium skin with Evo-Grip and filled the external holes very carefully with Sikaflex 512 caravan.  It's not the most aesthetically pleasing repair but it should do the job!