My experience with it is limited. I used some on an SOB trailer years ago. Although Kay's dad used it years upon years ago, and he was the one that recommended it to us. I would think it would be a real bear to get off, so I might be tempted to leave what's there in place, and maybe roughen it up with a heavy grit sandpaper. That would provide teeth for the new stuff to stick to. But, then you have the problem of having multiple layers and extra weight. I think no matter what you do, it's gonna require lots of elbow grease.
I don't know that much about Argosy's, so I can't 100% answer the seam question. I think the biggest difference between an Aistream and an Argosy is that the Argosy's have fiberglass end caps instead of aluminum, and they are painted. If the shell construction is pretty much the same, then you'll have a center roof panel connected to the upper side wall panels. So, one roof seam down the length of the trailer on either side, plus the seams where the end caps attach.