I recognize that the drains have been checked; but, my W124 had these symptoms and the problem was water retention in the rocker panel. The water stayed in the rocker until it got high enough that it backed up into the RF floor.

Find the 3? 1.5" round black plastic plugs along the rocker and remove them. On mine it was plugged drains but not the obvious e.g., sunroof ones instead it was the rocker panel drains.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer