The walls that separate the sections of the home above the ceiling are not exterior walls and are not typically insulated.
It sounds like your home has possibly; settled out of level or a shingle near or on the center line roof cap has been compromised. These would be the first things to check in the event of a small leak, near the centerline, of a recently installed home.
Settlement is not unusual on many soil types especially during the first year and might require a re-level of the home. A single loose shingle on the centerline roof cap could cause leakage and is a very simple fix.
State laws today require all home installers to be licensed. Call your installation contractor and ask him to come out to determine where the problem is and what it will take to stop the leak.
Clayton has a good reputation for customer service. If your dealer continues to ignore your problem phone Clayton's home office at 800.822.0633