It's not legal for the passengers to sleep in the RV while the RV is going down the road.
State laws say that all passengers of the RV including the driver must wear there seat belts while the RV is in motion.
However when I was a kid my dad had an RV and I used to sleep in bed of the RV while we were going down the road because he'd travel at night when it was my bedtime.
We never got into trouble and besides a cop or anyone else could not see you sleeping in the RV while it's going down the road so they won't know it anyway.
Still it can be unsafe to sleep without seat belts in an RV.
But if you get in a serious wreck in an RV the RV is gonna be destroyed anyway so whether you're wearing your seat belt or not you still may wind up really hurt or dead.