First of all, welcome aboard. If you are talking about the stuff that comes in a spray can, its a relatively thick black coating. Why would you want that on the outside of the trailer? I could see it if you had the inner skins off and wanted to spray the seams on the inside. And while we are on the subject, I used some of it to seal up a seam in a gutter on my house and it didn't work for very long, started leaking again. May have something to do with movement of materials, but I wasn't impressed.
When I first got my trailer, I focused on the seams around the windows and used the gray vulkem. I havent done anything to seal up the seams where the exterior panels meet and so far dont need to, no evidence of leaks. The PO of the trailer had used lots of silicone caulk on the rivets on the roof, I ended up removing all that and sealed up the roof with a material called ceramiflex. It's a white coating designed for commercial roofs and it does a great job sealing any potential leaks while cooling down the roof significantly during the summer. I detailed that job here: http://www.airforums.com/forums/f478...lex-98479.html
The other problem areas are usually the waste system vents on the roof and all the marker lights. Check all those for condition and recaulk with vulkem or replace as needed. The vents usually have a rubber gasket that degrades with time and lets water down in between the walls.