Just towed my Ford C Max 3300 miles around the Southeast. Only one tiny chip and I think it could have happened while driving.
But, and this is a big but, this is on a Class A gas motor home with a lot of overhang. The rear bumper is 12 feet away from the rear wheels so probably doesn't get a lot of rocks thrown up by the rear wheels of the motor home. I would expect Diesel Pushers to throw up more stuff since the wheels are a lot closer to the rear bumper.
2007 Damon Daybreak 3060, Ford 16,000# Chassis,
Ford C-Max Hybrid Toad , Suzuki V Strom 1000cc