I've just bought a second hand caravan and it was heavily mildewed; covered in loads of green stuff and mildew spots.
It took more than pure water and a brush to get it looking good; we resorted to using a bleach spray in the end.
I would also add that the roof, which is made of a smooth material was easier to clean than the sides which is 'textured'. My brush and pure water took most of the green stuff off, but the mildew spots needed cleaning 'traditionally' with a cloth.
I'd have wanted more than £20 to clean my caravan; it must've taken at least an hour-and-a-half; mabye longer, though it was the 'textured' sides which took the most work.