If you grab the door on the side with both hands, does it easily bend and flex?
These doors are constructed of wood frames, filled with styrofoam, and covered with plastic/aluminum. Over time, the wood can rot away and cause the door to sag or bend. If yours bends easily, you need to have some wood replaced. Extra weatherstrip may help, but I doubt it'll fix the problem. These doors didn't have very thick weatherstripping on them to begin with.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR