When riding in an RV you must have all passengers sitting upright in the seats and they must be wearing seat belts in the RV while it's moving down the road.
So the only amount of passengers who can ride in an RV is the amount of seat belts that are in the RV to secure those passengers.
So if pulled over and someone is sleeping in the RV without being upright and in a seat belt then you'll get a seat belt ticket.
It's very unlikely that you'll get caught though if someone is indeed sleeping inside the RV because no one on the road can really see that person sleeping in the bed in the RV but it's much safer anyway to keep the person in a seat belt.
Because if the RV had to stop suddenly then the passenger sleeping and not in a seat belt could be thrown around the RV or into the front of the RV and be injured or even killed.