Just our experience on our Alaskan trip....we pulled into Whitehorse Yukon and saw folks removing their stiff across the rear flap from their motorhome. We saw that they each had a layer of 3/4" rocks laying at the base of their windshield on their towed vehicle.
We just had stiff tire flaps behind each of them and no rear flap - stiff or brush. Our towed vehicle did not get any damage for the whole summer trip.
Later when driving the interstate in pouring rain we took note of motorhomes driving the opposite direction. The ones with the stiff flap were throwing an arc of water high toward the windshield. Ones that didn't have a rear flap were tossing water at the tire area of the towed vehicle.