Here is my plan for travel in and out of Colorado during the winter of 2015:
1) Open Hot & Cold faucets in both Kitchen and Shower and let the 12 V water pump empty the fresh water tank down the drains and into the grey holding tank. When the faucets all spit and bubble, the supply lines are fairly clear. Don't forget the toliet flush inlet water.
2) Turn off the 12 V pump and leave all four faucets open
3) Be sure you buy or make a fitting that adapts your City Water input hose inlet to accept compressed air. Wal-Mart sells a plastic air pressure-to-hose adapter for a few dollars in the RV dept. I cobbled a brass one together from parts sold at ACE Hardware. Cost about $10.
4) Apply moderate air pressure (about 50 lbs) from a filling station to the air-to-hose adapter. Make sure all the faucets spit and hiss. This time, also hold the toilet flush lever open so that the flushing water inlet hisses and spits too.
5) Dump the black and gray water tanks.
Tow the R-Pod "dry" in freezing weather. I believe that running the Hot Water heater for a few minutes 3-4 times during the tow day will keep the water in the HW tank from freezing.
Add some fresh water for Camping at night, mostly for flushing. With the furnace on all the water lines
should keep
above 30 deg F OK. Residual water in the three big tanks can get slushy, even freeze, without hurting anything.
If you will still be towing the next day in sub freezing temps, repeat the steps for winterizing with air.
Carry bottled water in the Tow Vehicle for drinking when camping dry.