Draining Water Tank Over Winter

.... or keep the heating on 24*7 when not in use?

That is what we do, our MH is stored at home on a hook-up.  We just leave the heating on with the thermostat at a very low setting, so most of the time the heating is off, but kicks in when temperatures get close to freezing.  The cost so far has proved marginal, it adds a bit to our electric bill, but way less than the cost of, say, a new mixer tap never mind a new water heater.