I think it depends on how close the rear wheels are to the bumper on the coach. On my Greyhawk its probably close to 5' so i dont think anything with weight (rocks n such) will bounce around enough to reach the toad. If the rear wheels are closer or stick out from the fenders there's more of a chance and if you travel down non-paved roads there's a higher chance. If you have the SAS version you also have some height advantage.
Obviously there's a market or there wouldn't be any to be purchased. I saw some net ones that seem easy to deal with versus some hard material. I'd probably get something like this before some additional mudflaps that are closer to what you are trying to protect.
Towing your car behind your housed sure is expensive