Black stains on the side of my van?

I am looking for help/advice on the best way to remove or prevent the black stains from the side of my van.

I purchased a new Swift Challenger Sport in February this year and have experienced streaks down the side of the van. The nearside is worst affected, however there are some marks on the offside. The black stains come from the awning rail downwards and are difficult to wash off. 

Do the awning rails on new vans come with some kind of lubricant in them? The stains seem to be greasy. As mentioned above, the nearside is worst affected. This side obviously takes the awning, but in addition, this side also gets the brunt of the weather whilst in storage. I have polished the full van previously, which removed the stains quite well, but after placing back in storage for a few weeks the stains are back.

Any advice would be greatly appreciated.