I seem to recall that on one of our caravans, many years ago, the seat backs were held in place by two flat strips of metal which were fixed to the seat back by screws. The metal plate was formed so that the lower part was bent to be slightly proud of the part that screwed into the seat. On the wall of the van there were two receiving plates. Each was screwed at either end, but the middle was formed to allow a gap. The seat was slotted into these by offering up the back, but slightly higher than final position. As the back was lowered, the metal strips engaged. To remove, the seat back was moved vertically upwards.
Worth a try.
Davy