Hi Ron
We have not had any issues with rock damage on our tow car, a 2014 Chevy Equinox. We rarely drive on anything but paved roads. When we do drive on gravel roads, usually RV parks, we're usually going about 5 mph so not an issue.
If getting a Roadmaster tow bar, make sure it's not one of the ones that have a recall. We purchased a new Roadmaster Nighthawk tow bar earlier this year only to be notified by Roadmaster that we were part of a recall that can easily be fixed... once parts are available. Still waiting.
Ron