Oh, Bob, you left yourself wide open on this one...
"I want to drain my fresh water tank so that I can sanitize it (the water in it now is three years old and is making my Jack Daniels kind of chewy and funny tasting).
The manual doesn't have anything to say about how to do this."
I usually ignore my Jack Daniels Manual on this one, and add no water whatsoever.
That issue aside, it sounds like the switch you are messing with in the water bay, assuming that's where your looking, is the one that toggles between adding water to the tank and the direct connection between plumbing devices and park water. You should have another switch in the water bay that's labeled "Fresh Water Dump" or some such thing, with Open and Close labels top and bottom. Though entirely optional, I highly recommend postponing the Jack Daniels lubrication until after this difficulty is resolved, despite any freezing weather you may be enduring.
That advice taken care of, the drains in the pump area are not likely to fully empty your tank. However, while you're in there you might want to unscrew the screen filter holder on the pump inlet and clean it before proceeding. As to sanitizing the tank, there is the method of hooking a hose to the "City Water" fill hookup and putting a cup of bleach in the hose, than connecting it to a spigot and flushing the bleach in while filling the tank, letting it set some hours, than dumping and refilling, yada, yada. Optionally, you may prefer putting the Jack Daniels in the hose, however it may not be quite as effective at sanitizing the tank as it does you.
A search of the Forum may provide a description of my tank sanitizing methodology that I find less messy and problematic. If I can find it, I'll come back with a link to it. Hopefully before this evening's happy hour.
-Joel