Re: How can I keep drinking/washing water from freezing?

Vodka you say?  According to the freezing point depression tables in my
trusty old CRC (Chemical Rubber Co.) hand book (1971 pg D-188) you'd need
about a 25% solution to lower the freezing point all the way to 10F. 
That's about 2gal of vodka in your 7gal tank.  50 proof wash water might be
interesting but perhaps a bit expensive.  I don't know how often you fill
your tank or the initial temperature of the water source you use, but you
might get away with 1/2 to 1 inch of Styrofoam insulation wrapped all
around the tank.  I'm not sure of the shape of your tank but using as a
sort of  worst practical case something shaped like a slab-sided old jerry
can, an admittedly simplified calculation using about 50F water to start
shows a time to cool to freezing of at least 48 hours for an inch of
Styrofoam.  You might also consider heating a gallon or two of water to a
nice dishwashing temperature and pouring it into the tank each night to
extend this time, especially if the tank has had plenty of time to cool
since its last filling.  I would think another real problem might be the
spigot or supply lines, if any, freezing.  This could be where a really low
watt heat tape and insulation of some sort would be useful.  I hope this
helps keep the dishes clean and the tank unfrozen.