Hard to tell from your photo of the rear of the rig with the rock guard...but it looks like the rock guard is touching/dragging the ground. That may be your problem...the guard may be picking up stones and flinging them back.
I'm sure others with rock guards will chime in, but don't have on on our Lexi and don't plan on getting one. Just have some factory installed mud flaps, installed directly behind the rear duals.
Towed our Subi over 12,000 miles now with no problems, rock chips or damage.
Steve & Cheryl + Charlie & Casey, our furry kids.
2012 Forrest River Lexington 283ts
Toad, 2013 Subaru XV Crosstrek