Get a can of "Ooops", at any store, and clean the area thoroughly with the "Ooops". It will take off a lot of crud. When you're done, wash the roof with soap and water and then rinse well and let dry.
Remember, the "Dicor" will lay flat and spread when applied. Try to apply evenly when resealing and give it a chance to spread out. It will make for a nicer looking job.
One of the spots that really needs sealing is where the front and rear caps meet the coach body. Most companies run Eternabond along the top for the seal, but the point where the side walls roll over the edge, they usually use a clear sealer. I believe it's called Flexcoat, but it's a clear sealer that will hold up for several years. Clear silicone will work too, just apply carefully because it's visible.