First of all, if the black and gray tanks are drained, they don't need any antifreeze. Don't know where you are at or how cold it may get, but if you have power, could you run an electric heater in the rig? Open cabinets that are under sinks so those pipes get a little heat.
One last thing to keep in mind. Water does not instantly freeze the minute the air temperature hits 32 degrees. If the rig is in the sun, and it makes 60 degrees inside the rig during the day, could probably get to 20 at night and not freeze the pipes, as long as the temp came back up the next day. That's my two cents, others will have a different story.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT