The thermostat on your tank-style water heater continuously attempts to maintain a specific temperature based on factory settings, whether you’re using hot water or not. So, if you need to conserve propane, then you shouldn’t keep your water heater on all of the time. If you’re using the water heater on 120-volt electric, it might not be as much of an issue to keep the system on unless you’re exceeding your RV’s power capabilities while using other appliances.  Tankless water heaters are meant to be left on as they are very efficient and don’t light the flame unless there is a demand for hot water when a faucet is opened.