Before going full timing, I followed many RVs in the rain (Portland, Oregon area) and noticed what Mile High's daughter did. Full side to side rock flap just 'sploshed' the car with buckets of water, and whatever other debris the RVs tires picked up. The side to side skirts flooded the toad with smaller droplets, but still a heavy rainstorm. I think this has to do with the flaps acting as a sail as mentioned earlier. The best rigs to follow ONLY had the OEM mud flaps behind the duals.
So that's what I've done, is use only the OEM mud flaps. Not using a tow bar skirt either (far too expensive for some nylons and plastic tubing). No damage to the toad so far, and only time the car gets filthy is when I'm in and out of 'scattered showers' during a trip, or have to detour on a gravel road. It does show some dirt generally after a few days towing in fair weather though.
One thing I did do to the toad is cover the headlight lenses with that
Clear 3M stick on protective plastic sheet. And so far it has saved one lens from a rock hit. I can see the evidence in the 3M material...lens is fine.
Just measured the mud flaps and they are 1/2" from the ground...but, it's possible the rig raises a little when the air bags fill? I've not noticed much of a rise but it probably does.