IF (note: big IF) you leave your inverter on when you drive (I do) and IF your engine driven alternator has the capacity, try the following: buy a line voltage thermostat and a hair dryer (most any hardware store). Install them in your water service bay, set the hair dryer to "low" and set the thermostat to around 50 degrees F.
On your 1st few trips, you may want to leave a thermometer in the bay as well so you can check the temp when you stop. Adjust as necessary.
Of course, this will also work if you are camping in a freezing area and have elect. hookups available.
Tom & Jan
Full timers since April '06 - 1 fur kid (Archie - mini Poodle.)
2006 Beaver Patriot Thunder towing 2012 Subaru (4 down)
Started workamping Sept '07 - "This isn't too bad. Think we'll do it some more."