We had our 40' motorhome for ten years and even took it to Alaska and down gravel roads for boondocking spots. Our towed vehicle never got dings from towing.
We only had flaps behind the wheels.
On our Alaska trip we actually saw folks removing the stiff long rear flaps. We looked at the vehicles and they had a layer of large stones laying at the bottom of their windshield. The stiff flap was actually throwing the rocks at the car.
On another trip we were around Salt Lake City in heavy rain. I started watching motorhomes towing in the opposite lanes. Those with the big flap had a heavy spray of water hitting around the hood of the car. Those without the flap had the spray of water hitting around the tire area - much lower.
That convinced us that the big flap is just for looks unless you drive on pristine roads.