Most newer trailers have an electric element for the heater, that should handle most need of hot water. It is controlled by a thermostat. The switch for the water heater that lights up, is only telling you that the gas part of the water heater is ready to work if there is a heavy demand for hot water. Usually when you have a shower. You can turn the electric element off, but they gas would have to stay on to get any hot water.
Since most of the time you are at a campground when water heater is on elec, there is no extra charge to you. On gas you have to refill propane tanks more often.
When I am at home and trailer is plugged-in, I turn both the elec. element and the gas off.