The last time I looked into this, the "general law" was:
Children MUST be seat-belted - no exceptions. Child seats and boosters are applicable as required in cars.
Adults must be seat-belted if the seats have seat belts. Meaning, if the dinette seats do not have belts installed, then adults can sit there without seat belts. The cab seats MUST have seat-belts, the rest of the coach is optional.
Another thing to remember is that even if the seat has a seat belt installed, it doesn't mean it is a properly functional seat belt. Many folks added their own seat belts, but just screwed them into the seat base (versus bolting them to the frame).