We full-timed 8 years in our motorhome with only mud flaps behind the tires - no full rear flap. We traveled constantly, boondocked a lot on gravel roads and spent the summer in Alaska. We had absolutely no damage to our Jeep from towing.
We were in Whitehorse in the Yukon and a couple RVers were removing their full rear stiff flap. They showed us a layer of 1/2" rocks laying at the base of their windshield on their cars. We had just traveled the same construction area with no damage.
Since then others have mentioned that the full flap is worthless.
Later while coming through Salt Lake City in a heavy rainstorm I made it a point to watch opposing traffic and motorhomes. Those with the full flap had an arc spray of water aimed high at their hoods. Those without the full flap had the water at tire level. That convinced us.