I was told the brush type of skirts actually kick up rocks. I bought a skirt that goes from the RV bumper to the corners of our towed car. Made by Protect-A-Tow, it has lasted over ten years, and around 50,000 miles. It uses a bungee rope around the perimeter to hold the tension, which I had to replace once.
The theory is to keep the rocks down, and let them roll under the car. For years, I kept a bra on the car, too. (Is that like belt + suspenders?) The current cost is $220. Mine is growing thin, and I'll replace it with another.
Here's the link:
Protect-a-Tow