We have a class A and our kids are all teenagers now, but when we first bought it, they were ages 1, 3, 4 & 7. Yes, in those days, we did use at least 2 cars seats with the youngest ones. But once they were big enough to be belted in, we no longer used the car seats in the motorhome.
Our general rule was, once they were old enough to move around on their own, that IF they needed to use the potty, they would ask permission. If I felt it was safe (I was driving), then I gave them permission. This would require that we were not in a high traffic situation, there was lots of space between vehicles, good weather, etc. etc.. In other words, a low risk situation. If we were in bumper-to-bumper, stop & go traffic, then we pretty much encouraged them to get up and use the potty during the traffic jam. Again, very low risk situation. Once we started travelling at speed, most of those caught in the traffic jam were pulling into rest areas to use the potty while we just kept trucking down the road.
If we were on a long trip, we would have premade sandwiches, snacks & drinks and wifey would distribute these. Or if a diaper needed to be changed, we'd pull over at the next convenient rest area. As the kids got older, they had more freedom, but they will ALWAYS ask for permission to make sure it's safe, and so that I know they are no longer in their belted seats.
In my opinion, there is no better way to travel with children than in a motorhome. Use it wisely and enjoy the trip!
~Rick