The on-off switch turns the electricity on to the water heater. It has nothing to do with filling the tank. You probably have a bypass valve located inside on the back of the water heater. This valve must be positioned to fill the tank and set to bypas when winterizing unless you want to fill the tank with anti freeze. No need to empty the tank after each trip unless it is a long time between trips. No not keep your black tank drain open all the time. Allow the tank to fill --at least 2/3-- and then empty. Your blavk tank flush should be located on the outside of the TT somewhere above the drain valve. Looks similar to the city water hookup. The 2 drain valves under the trailer are your low point drains used to drain the system for winterizing or other times when you may want to drain all the lines. They do not drain the water heater--you must remove the plug on the outside of the water heater to completely drain the hot water tank.
