You'll need a nice dry day to remove your awning rail. Its a long job but quite easy to do. Some van rails come off in two strips. Front half and rear.
Keep the screws handy as you will need them for the re-fit and they should be of stainless steel. If you need to buy screws be careful with the size spec as the screw heads are smaller than the screws you buy from the DIY stores and most of them are plain steel which will rust!
When the rails are off they will need cleaning off and maybe needing new rubber laces as they tend to dry out and go brittle. The van will need a clean off as well. Most sealants come off using white spirit after the bulk of it has been picked off using a plastic scraper of some kind.
You can buy the sticky mastic tape in various widths to re-seal but I prefer the Hodgsens seamseal CV. If you use it you will need a mastic gun. The exstra long one as the tube it comes in is longer than the usual sealant gun. Squeeze it on about a 12mm and put a long headless nail into a few of the screw holes to aid the refit. Its a lot better than trying to hold everything in place while you find the screws!
After the screws have been replaced, clean the unwanted goo from the rail using the same plastic tool and a bit of white spirit on a rag. Job done!
Good luck with it.
Unfortunalely awning rail removal is an all too common task as it's a well known area for water ingress. I would add to V6Steve's excellent post, when you have cleaned all the old residue off with white spirit, it is likely to leave a fulm of white on the job, hence the name. This is easily removed by wiping it over with metylated spirit to give a nice clean finish for the new sealant to stick to.
Dave.
------------- Give a man a fish and he will eat for a day,
Teach a man to fish and you can get rid of him for a whole weekend.