my theory is the best place is the belted bench of the forward facing dinette seat (if you have one)....with the table in "bed" mode
as far as I remember from the not so long ago years dealing with car seats and boosters...these things are all intended for use in forward facing seats...either flipped around rearward for the younger kids and forward facing for the older.
They aren't set up for side facing. Most don't even have decent side impact protection.
We did at times set up our kids in booster seats on the sofa but not often. Normally they would be at the dinette. And most of the time we'd have the table up in eat mode but I was always aware that it makes a pretty good head knocker in the case of forward collision.
In bed more the head knocker table is out of the picture.
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