Hi Dave,
Welcome to the Sunline Owners Club. I had a T-2251 which also had a metal roof. My 2007 T-2553 has a rubber roof. I'm not sure when the switch to rubber happened. Any RV dealer in your area should be able to suggest something to seal the roof. Check in the Tab section at the top of this page for suggestions. Also there are many people on this site who can offer suggestions. I've used a material called dicor to seal around vent pipes and other roof openings. Again, welcome!