I replaced the roof on one of my slides on a previous trailer. It won't take but 1/2 hour or so to remove the old roof. Use a sharp knife and cut around everything and peel it off. You will want to inspect for any water damage to your roof decking anyway. You also will need to remove any fittings on the roof like, trim, skylights, vents etc. Almost easier to replace them rather than trying to clean them up and re-using them. I wouldn't leave the old roof on and go over it. There would be no guaranty the new one would would stick. When you found out it wouldn't it would be too late.
B.O.