My mom always told me it was not polite to call people stupid, this is a rule I still abide by.
Seatbelts in RVs, trains, busses, and the like are a real grey area. What people do is pretty much their business and the law would agree as in most states it is a secondary offense. All front passengers have to wear their belt, but the back seat/living area of a MH are completely different and each state regulates their own seatbelt laws. I don't think this is a moral issue like all would make you believe. I travel with kids and have a total of 8 seatbelts. The certainly do not use them 100% of the time. Nor do they always look both ways before crossing the street or eat healthy 100% of the time. My job is to mitigate by utilizing safety to the maximum extent possible.
A good read......
Seat Belt Laws per StateWW