If you have heat inside, you probably don't need to leave the water heater on. BUT, make sure the water heater is in a space thats heated. i.e. if its hidden behind some drawers or in a cabinet. take the draws out or block the cabinet doors open so the inside heat can also get to the water heater.
leaving the water heater on would definitely keep it from freezing, but may not be necessary.
2001 Coachmen Mirada (Ford F53 6.8L V10) - Toad 2003 Saturn Vue
It won't do MACH 2, but I can get a sandwich and take a pee.