Funny you bring this up as my sisters '77 tradewind (Formerly mine) has Line-X on the front of the trailer(came with it when we bought it), and we put foam pipe insulation on the exposed lines for exactly that-rock protection on gravel roads.
It worked great, however I would probably prefer a traditional lower rock guard over the Line X due to its permanency.
The routing of the gas lines on the outside is to prevent gas from building up in the belly pan should a leak occur. Being outside, the gas should be able to escape (relatively) harmlessly into the atmosphere. If it built up in the pan, it would migrate its way upward and with the first spark. . . instant northern lights!
Be sure to leave the ends of the pipe insulation a few inches away from where it enters the belly for the above mentioned reasons.