On my previous truck and previous camper I had both mud flaps on the wheel wells and a Rock Guard that attached to the "bumper" hitch. I never knew how well that combination actually did work to help protect rocks and stuff from hitting the trailer until we had a winter trip. The photo below was taken right after we crossed (going Eastward) the Appalachia Mountains in West Virginia and finally got out of the snow.
The evidence on the trailer spoke volumes to me. Judge for yourself: You can see the Rock Guard in the first photo on the back of the truck:


The thing I DID NOT like about the Rock Guard was the fact it attached to the hitch. When removing the hitch from the receiver, it had this 8 foot wing attached to it, making it very, very difficult to store anywhere, especially if removing it at the camp site.
Next truck and next trailer I got an Equal-i-zer WD hitch and the Rock Guard would not work with it. But the new truck does have mud flaps, which helps a lot.
So, if it makes any difference with mud flaps only or both, here's a couple photos from this March (2017) when we took off and busted through some snow and gunk with our current trailer and truck (no Rock Guard). Here again, judge for yourself:


With the Outback TT and no Rock Guard, but mud flaps on the truck, after almost 4 years now, I don't see any rock "dings" on the trailer nose camp. (personally, I don't think the Rock Guard is necessary when you have mud flaps.)