You can get a "special" brush to clean out the awning rail, like this
Make easy work of cleaning your awning rail channel with this 'high quality' brush.
www.ebay.co.uk
But a standard bottle brush, bent a bit so you can slide it along the rail works just as well. Cleaning the channel out is the first task before washing the caravan.
You can get some awning rail protector, which is like the beading used to attach an awning with a strip of canvas attached to it. Like below.
Maypole MP951 12M White Caravan Awning Rail Protector at the lowest UK prices from Euro Car Parts UK’s No.1 for Car Accessories, Car Styling & Car Care. Buy today Online, In store or by Phone + Branches Nationwide – Next Day UK Free Delivery.
www.eurocarparts.com
I have not used the stuff, but there are a lot if places selling it, so I assume it works