I would suggest a least to most direction. As said, clean out the groove with a small screwdriver or pocket knife. Squirt a small amount of Elmer's glue in the groove. Push the molding back in and secure with some painters tape. Clean up any glue that oozes out with a damp paper towel. If the Elmer's doesn't hold, then progress to a stronger glue, maybe a rubber cement. The new glues work great, but for a beginner, or less handy person, the older stuff is more forgiving.
As the duck said, any handyman from the phone book should be able to work on the box part of your coach.