As others have mentioned, there isn't really a great option. Everyone I talk to with kids who RV have their own opinions and I'm sure each law enforcement officer you speak to will have their own as well.
We have 4 kids age 9 and under. We are typically on the "err on the side of caution" when it comes to our kids and carseats. What we decided to do (probably not legal in most states) was to create a new bracket for the seat belts at the dinette so all 4 of our kids could sit in their carseats at the table.
2 of our kids and their carseats weigh less than one adult, so my thought process was that if Thor thought it was OK for an adult to use the one car seat, it would hopefully be enough to keep the 2 kids safe per side. Our bracket was custom made by a professional machine shop and is much stronger than the stock one.
Is it a hassle to remove the cushions and install the carseats every time? Yes.
Will it keep them safe if we are in a horrible wreck? maybe, maybe not...
Do I believe it will atleast give them a better chance of staying safe than just roaming around in the RV? Yes.
Old Single Seat Belt Bracket
New double Seat Belt Bracket - still mounted to the floor.