What people do, and what is legal (which are not the same things always) vary between people and states. In some areas, certain laws regarding car seats and seat belt use and such don't apply to motorhomes; it depends on how the laws are written. Often car booster seats are only required where suitable places to put them are available in the vehicle, so as not to entirely prohibit children from ever riding in antique cars and so forth. A motorhome is generally not classified as either a passenger car, a truck, or a van, but is its own category.
As a practical matter, many people do get up to use the restroom or grab a soda from the fridge while en route...obviously only if someone else is driving. Many other people avoid doing this.
In terms of safety, no motorhome is all that crashworthy, really. The seat belts they're equipped with would be of use in a sudden stop or minor accident, but I think would not be too helpful in a severe crash as the house structure often doesn't survive very well and, even if it does, there are just too many things that can get loose or fall out of cabinets or whatever and become missiles. The good news is that care and caution in driving will allow one to avoid most accidents. This applies to both class C and A motorhomes in general.
I don't have any children of my own, but if I did I suspect I might take a rather permissive approach here: remain seated in general, but using the restroom or getting a book etc. is okay with the driver's permission (given based on the road conditions). Others have different opinions, possibly wiser ones.