I'm with downtheroad... if its a permanent site, consider purchasing a carport type structure or building some type of roof over the entire trailer. That will keep the entire structure cleaner and more protected from the elements.
Another option is to simply add awnings over your slide, which will protect them.
I have seen (though I didn't get close enough to inspect) people using the corrugated plastic roofing over their slide outs. From what I recall, it was mounted just underneath the drip rail that goes over the slide. Not sure how it was attached though.