I got my flaps when I bought the truck in 2006 and they are as good today as when I bought them. I think I paid $15 for the pair. Even with 19.5" Wheels and tires, (34" tall), they reach down far enough that nothing of weight is getting slung up and hitting my camper overhang.
I gotta wonder about those flaps mounted WAY out there away from the truck on the hitch extension. Any rock that got past the wheel well mounted flap is going to be traveling pretty shallow and most likely will not be far enough off the ground to damage anything being towed.
A side photo of a truck, those flaps way out to the rear of the camper, and the towed trailer with the 'rock' zone shaded in gray would show it's unlikely a rock could get that far back there and still be high enough to cause any damage.
Mine;


