I reseal my van every 3yrs last time I used a diferent product than the usual mastic type sealent, I only did one awning rail as it turned out to be very messy and I could not get a good smooth finsh.
I remove the awning rail some of the screw heads may be rusty a good tool to use is the dremel with a small cutting disc to cut a new slot in the screw. Its best to replace all the screws with new ones, remove all the old sealent using scrapper and white spirit fill the awing rail with new sealent ( cheapest place I found for the sealent is JR Leisure) refitt, the excess sealent will be forced out as you tighten the screws. Use a scraper to remove and finish with a rag socked in white spirit to give you a smooth finish. Use the sealent on all joints, I find the best sealent to use around the roof hatches is Evo Stick Nail and Seal it so good you may never have to do it again but as before it is difficult to get a smooth finish which is ok on the roof no one sees it.
Its an easy job to do, may take you a day to do but at least you know its been done right and you have saved your self a few hundred