My wife is a child seat nazi and I am - well, not. We went though this a lot of times when we had our first child. She researched and researched and like you mentioned, there is just not much data out there. In the end we took the car seat and installed it on the couch with the seat belt that is mounted to the floor. This was very secure and worked well. The only negative is that the seat is facing sideways, not front to back as is recommended by the car seat manufacturer. It was pretty much making the best of a bad situation.
I've seen pics of RV crashes and my personal view is that if the crash is at all serious, we're all hosed - seatbelts, carseats or not.