Got it! Sometimes knowing there is a solution is the major breakthrough. All your comments made it clear there HAD to be a way to drain the thing.
There is. What looked like a smudge on the plumbing schematic was, in fact, the drain outlet. And the drain hose led to a compartment under the forward seat in the dinette; the electrical stuff is under the aft seat. Makes sense. And the valve didn't look at all like what I was expecting: it's a small version of the "slider" valves which control the flow of waste water from storage tanks to the great outdoors.
The exit route under the coach is a very anonymous short piece of black tubing; I was looking for something that looked more like the drain plug for crankcase oil.
So not only did I find how to drain the fresh-water tank, I found the water pump -- wondered where it was; good to know in case I have to fiddle with (or replace) it.
That's actually the last of the major mysteries. I shudder to think what it would have cost me to take it to a dealer and get these questions answered.
Many, many thanks! In time, I hope to be a source of information, not just a consumer.
Rich