While many of the lower end travel trailers have very delicate roof structures, your big ol' HR Presidential should have a very stout roof. Unless there is water damage and rot, then you could probably play basketball up there.
I'm guessing you have an aluminum roof rather than EPDM rubber so I am trying to picture what you mean when you say, "replacing the seam down the middle". Can you explain a little better?
Also, have you heard of "Eternabond" sealing tape? Highly recommended. Perhaps that is all you mean - that is you want to run a sealing tape along any seams, pipes, vents, etc.