When the National Highway Traffic Safety Administration (NHTSA) and the Federal Motor Vehicle Safety Standards started requiring first seat belts and then belts and shoulder harnesses all builders complied. Before they were mandatory they were "recommended" and some builders complied and others did not. No fines involved for not putting them in when recommended, but big ones for not having them when required. If you don't have them and there's no evidence that a subsequent owner removed them, they were not required.
2013 Tiffin Allegro Bus 43QGP
2011 Chevy Silverado 1500 Toad