Black water tank completely full and won't drain. Help pls.

bertschb,

In theory what you're saying makes perfect sense. BUT if the valve isn't opened, then this device won't work. OP has to figure out (which it sounds like he did) that the valve isn't opening. If it isn't, think Cuminsv12 solution maybe best option. Otherwise, I wouldn't have a clue as to how you'd get valve open w/ full tank. I for one certainly wouldn't want to find out the hard way.