Odds are the sidewalls are vacuum-bonded sheets, with the exterior a sandwich of filon and luan plywood and the inside a sheet of wall-papered (or vinyl) plywood. All glued together and glued to the metal structure as well. You can't get at the outer plywood from the inside and vice versa. So, the first question is which side is damaged (or both)?
For vertical seams (what cracks?) use any good grade of exterior caulk, e.g. Geocel Proflex RV or a window & door caulk. For the roof seams and around roof openings, use Dicor or Alpha self-leveling sealant. If you prefer, you can apply Eternabond of Dicor tape. You would need to remove old caulk first to get a clean flat surface for the tape. Also, the tape can be difficult to apply tightly to curves and around openings. Sometimes the self-leveling sealant is a lot easier.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC