Thank you to all who responded with advice and/or experiences. We did finally solve the problem, partly through guesswork and finally with the help of a Cedar Creek service rep and the previous owner.
Our unit was originally built with 3 pull handles, one for each tank - galley tank, grey tank 1 and the black water tank. It was obvious that we were short one pull handle, thought it was for grey #1, and that it might be 'hidden' somewhere.
Long story short - the previous owner had the black water tank pull handle removed and had a valve installed at the end of the sewer outlet instead. It wasn't the grey tank 1 pull handle that was missing after all, it was the black water tank pull handle that was gone. Cedar Creek confirmed that this was not unusual. After I figured it out the previous owner returned my somewhat frantic phone call and apologized for not explaining how the modification worked. It never occurred to me to ask!
Kudos to the previous owner for responding to my call and kudos to Cedar Creek for getting back to me right away even though our unit is a 2008 model.
I'm just happy that we can now use the shower without worrying over how to dump the tank.
Moral of the story - when asking about all the cool topside features and how/if they work - don't forget to ask about the 'bilge' features. ๐
