On our Alaska trip we pulled into a campground after a couple others. When we all got to our sites the others were taking off their full stiff flaps from the rear of the motorhome because they had a layer of 1/2" rocks laying at the base of their towed vehicles and dings on their hoods. We had driven the same gravel road they did and no damage.
We just had flaps behind each tire, not the full one at the rear, and received absolutely no damage the whole trip or ever during our travels.
Later when driving the interstate in Utah during a heavy rain and for lack of something to do, I started noticing motorhomes going in the opposite direction. Those with the full width flap had water arcing high on the hood. Those without the flap had the water aimed at the vehicle's tire level.
That convinced us never to have that 'free advertising' across the rear on a flap.