There is a lot of angst on this subject. Ideally all passengers really should be belted, but it is not required for those outside the driver area. Buses are not equipped with belts other than front seat, even school buses. There are various reasons for that, some practical, some based on accident statistics, and maybe a few that are perhaps self-serving excuses.
Safety experts will testify that occupants in larger vehicles are much safer than those in smaller lighter ones. Size and weight is an advantage if colliding with a smaller vehicle. It even helps a little if colliding with a bridge abutment or another large vehicle. Even a severe case like a rollover is more likely to result in bruises, cuts and broken bones than in death. The net is that occupants in a large vehicle are a lot less susceptible to severe injury in an accident. On the downside, an RV is full of corners, edges and objects that can cause a lot of hurts if the passengers get thrown around. Common sense is that seated and belted passengers are going to be less vulnerable.
Having survived a non-trivial motorhome accident, I can tell you it is true. However, large covers a broad range, and a 25 ft Class C or B-van has more vulnerability than a 40 ft Class A. Even though the van-based RV has the engine in the front. Height and weight are the main factors, and a 12,000-14,000 lb class C still dominates over a 3500-4000 lb SUV.
