OK, I think you've answered my question. My grey tank was about 1/3 full when we got here and I expected it to go to empty right away. It didn't. But today, it shows empty so I guess it has to get to a certain level - maybe 1/2? - before it drains. That's ok with me.
Thanks very much. I really didn't want to connect the big hose that I keep "just in case".
Peg