Most of the ducts should be ridged. The cross over between the homes sections is probably flex. Expansion would be the most likely explanation. There could be a disconnect or a section that has come loose along the duct system. Most of the time you can inspect duct work by putting a flashlight inside one vent and using a mirror in the next vent in line to see what's going on between.
Repairs in the ceiling are more difficult then in the floor. Ducts in the floor can be accessed from under the home. To get at ducts in the ceiling you have to go in thru the roof ***NOT RECOMMENDED*** or thru the ceiling. Repairing the duct most times is a simple matter of pop riveting or taping. This leave you with the problem of cosmetic ceiling repair. See this video
, This Old House general contractor Tom Silva explains how to repair a plaster ceiling.
"Homes that don't have sheetrock ceilings probably have tile or ceiling board. The ceiling tiles are generally 16" or 4' wide and run the width of the home. Some types of ceiling tiles are held up with screws and rosettes. Other types are screwed up at the seams and then the seams are covered with a plastic spline." If this is the case in your home see Manufactured Home Ceilings - Repairing or Rebuilding
by Mark Bower