Why not just stop at a rest stop every couple of hours?
Spoken like someone without kids! :)
We've had our RV about 9 years now, and when we got it the kids were 0 and 2. They are now 9 and 12. So they have grown up in the RV.
RVs are great when traveling with kids because invariably kids have to pee every 10 minutes going down the road. Or they need a diaper change. With an RV, they can go potty whenever they need to and the other adult can help if needed without having to pull over and find a bathroom.
The rule of thumb is as others have stated: Most of the time you should be in a seat with a seatbelt on. If you need to get up to get a snack or drink or potty that's fine but otherwise you get in a seat with a seatbelt on.
Sometimes when we are driving late at night I will let the kids climb up in the overhead bunk and sleep. I don't like it but when it's late and they are sacked out there's not much other choice.
I try to keep passengers seated in the inbound seats instead of the outbound seats as the side of the RV could get sheared away in an accident. I worry sometimes about the kitchen table breaking loose and becoming a guillotine to those seated at the kitchen table but it is what it is. Most of the time the kids prefer to sit in the bucket seats away from the table anyway.
Our kiddie car seats fit at the kitchen table seats just fine. Again we positioned them in the inbound part of the benches away from the side wall of the coach.
Steve