I am wondering how effective those broom style thingies are that hang below your hitch and parallel the bumper?
I was told, by one who had used these, that they do not work very well. He said that they tended to fly back due to air flow while moving, thus exposing the trailer to rock damage. Sure enough, I observed exactly this effect while watching passing RV's on the highway. Something much stiffer seemed to be needed.
I made up a extra full width "mudflap" out of old rubber conveyer belt attached to a removable square tube bolted to brackets on my hitch mounts. This was quite stiff and did not bend too much due to airflow, which protected the trailer well. The bottom edge was 2 to 3" off the road surface, but you have to be careful with this clearance on rough roads. If the bottom edge of the flap actually touches the gravel, it will spray up rocks and become a source of rock damage. If you are running on rough rutted roads, more clearance will be necessary. The rear overhang of the tow vehicle also comes into play here, I was able to have the flap closer to the ground because my Explorer had a short overhang. Changes in grade could cause the flap to drag, and this becomes more pronounced with longer overhangs. The key is to try place the flap as close to the rear tires
as possible and as near to the ground as practical.
Unfortunately, I have recently changed tow vehicles so this fabricated rock guard will no longer fit, and there seems no easy way to implement a replacement due to hitch and exhaust pipe location. In the mean time, I have been avoiding long gravel road trips.