My tank is under my bed and the entire tank is accessable. It is sitting in a "tub or well", with half the tank below floor level. My valve is on the bottom of the tank down in this well. I'm sure your's isn't the same, but if you have access to the tank, its probably there.
When I first got mine, I had to look under the coach for the drain line then traced it to under the bed. All the manual ever said was to open the low point drain valve, with no indication where it was