Question for the food truck / trailer people.

I purchased my truck and I am now getting ready to start building it out (photos will be posted as I go along).  I pretty much have everything planned out, but there is one thing I am not sure about and haven't seen on the boards that I can remember.  What do you do in the colder times of year to keep water from freezing ?  I understand that fresh water mounted inside can stay warm from kitchen heat and can be drained at night, but what about under vehicle mounted waste water tanks ?  Heat tape ?  Some kind of block heater ?  Sterno cans ?  lol.   Just wanted to get solutions from those that have gone before me.  ;)   Thanks in advance for your input.
 
Edit:  Also, what size fresh water tank are you guys using, and do you wish you had more / could do with less ?