There are lots of solutions. On my trailer, I built my Wraparound couch with a pedestal table.
A very simple solution would be to attach a piece of material ( steel, wood, or heavy plastic) to the back bottom edge of the filler piece.That would slip under the main table and prevent tipping.
Another idea would be to drill a small hole on the edge of each bench and install a small barrel bolt on each end of the filler.
Of course you could be sure and plant your backside further back from the edge when you sit down!