You should do what you can to sterilize it. We always take bottled water for drinking. Just more convenient. If we don't have water hookup we still use it for showers, toilet, dishes, cooking, etc. We are frequently on boil water orders at home and still use the water. Probably not as high risk as what a lot of people believe.
This is a photo of my tank when I had it out. It's sitting on the end. What you see is the drain line/tube, and the other hole is for the pickup line to pump.
Look closer and you will see the fill tube and the breather/over flow tube on the bottom of the picture. That is the top of my tank. Mine fills "all the way" to the top.
Lloyd, I see you have the Mod for the tank pickup. So does it stand to reason then if there is enough water in the tank to start draining when you unscrew the cap, should there not be enough water in the tank for the pump to start pumping. I ask because it seems like I have to put a lot of water in the tank before the pump will start pumping water. If I remove the drain cap water will come out rapidly but no water from pump until I put in more water. I also have the Mod in mine