As threads get longer there tends to be a departure from the original thread intent. This thread is relative to movement within a class A while in motion.
The absolutely safest way is to have all occupants in their seats buckled up while the coach is in motion. The reality for those of us who travel in class A motorhomes is there will often be some movement while underway. Whether to go to the bathroom, tend to a pet,or fix a snack. I think most of us exercise prudence and do not permit movement in traffic, congested areas, road construction, or bad weather. Or will pull into a rest area, off the side of the road or an exit.
When our children were young, they were belted while underway no exception.