We are planning to stay in MH this winter. Do I need to leave the water heater on all the time to keep water from freezin or can I just do like I do now and just turn it on when needed ?
Or to put it another way: You should be WAY more worried about water freezing in the other pipes/hoses than in the water heater itself.
The other side of that coin is: Having the water heater hot should help keep other pipes from freezing and once it gets hot keeping it that way shouldn't use too much fuel.