Normal mud-flaps right behind the rear tires is the best defense. They tame stuff flipped up from tires early.
Rear bumper area flaps can be wickedly brutal to the front of a toad. Especially if you travel gravel/dirt roads, though the same kick-up happens on paved surfaces. Longer rear overhangs cause more vertical travel, so a bumper/hitch flap would have to be very short to keep it from touching down. Think about those road sections that give oscillation or just whoopty where you know the bump stops are engaging ... Do you really want something touching down to flip up everything it can as it grazes the surface?
Some DPs need rear flap for proper airflow and IMO... any other use of rear hitch/bumper flap, that hangs down far enough to touch sometimes, is a flag that says: look at this gimmick I fell for
In contrast, I've see a few folks with a rear flap way up off the ground (looks sort of funny really) but those folks are smart, just taking an extra layer of precaution for what might get by rear wheel flaps. Not for me to say what overly cautious is...
We've pulled a wrangler behind every motorhome and learned the rear flap lesson the hard way!