No correct answer. An unprotected outside water line is first to freeze - most will disconnect that in very cold weather - no big deal. The stuff many focus on is inside the rig and whether that gets damaged depends on whether the rigs occupied (heats on), how the interior water lines are routed and whether those compartments have access to heat.
An occasional severe drop in temp won't usually result in a problem if the rigs occupied and you have heat on ... but if the rig isn't set up for winter and the temp is extreme/prolonged you may have an issue - probably not a concern this time of year.