In the later models of the MH (325, 345) there are seatbelts in the dinette as well as the barrel chairs. The barrel chairs that are bucket shaped (pre 82?) will not hold up to the stresses in an accident. I just pulled the upholstery off one and it is a hard foam material that is not very structural.
Seat belts can be added, but there are a few things to consider. Will the seat take the stress? Can I anchor the belt to the frame of the coach or just the plywood floor?
Belts have been added by some forum members. Belts are good. Most MH owners will admit moving around the coach while the coach is moving and my wife's favorite place to spend time while on the road it asleep in the bed. Is it safe, not really, but I think that some of it depends on where and when you do things. In town, on congested streets she is normally belted in and being my navigator. On open roads where I have trouble getting lost she feels safe enough to sleep.
You will get many different responses to this question. It has come up before. I would say the right answer is always wear the belt. What you do is up to you.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato