The way I look at it road travel in/on any vehicle is inherently dangerous due to the speeds involved and the other vehicles out there. We run around 40,000 highway deaths annually in the US. Government crash ratings are based on controlled testing which will be drastically different from the chaotic events that happen in the real world. I think how one drives is by far the biggest factor in how safe they are on the road, regardless of what they are driving. I'd also say vehicle condition and maintenance is very important. A front tire blowout, for example, will be many, many times more likely on an old weather checked tire, an improperly inflated tire, or a tire of incorrect load range. Handling is also important to safety as are brakes - how often do most drivers practice emergency stops or other emergency maneuvers in their motorhomes? An overloaded vehicle isn't going to stop or handle safely.
Structural integrity of the vehicle is a big factor in safety. One reason people die in tour bus rollovers in this country is the windows are mandated to pop out so the passengers can escape a fire. Not good when the passengers are unbelted and the bus rolls over. However, buses are required to not crush - which motorhomes are not. If a motorhome rolls and someone is in the back they are in trouble for sure. Motorhomes also aren't required to protect the rear passenger area in a collision. A car impacting the side of a motorhome would probably penetrate most of the way through.
As far as belts my understanding is they should only be used in forward and rearward facing seats - not longitudinal seats. I think this has something to do with what happens to the body when accelerated sideways while being restrained by a belt. I haven't researched this but I'd suggest anyone using belts in longitudinal seats investigate further. I also understand that rearward facing seats are the safest - but some people are more apt to get motion sickness in them. Probably the safest frontal crash position available in the coach of a typical Toyota based motorhome is prone with feet facing forward and braced - also pretty comfortable if you happen to have a couch! I do think the single passenger pedestal mounted seats are pretty good for kids. The sides nest around the child and with the belt on they are pretty secure and shouldn't be launched into anything during a hard stop or swerve. Make sure your microwave is fastened securely!