Water was pouring out because I discovered that the valve on the bottom of the fresh water tank was open. But these were also leaking. They appear to be some sort of diverter valve?
I can only assume that the valves on the fresh water tank drain will also leak (I didn’t try much more, I just immediately turned off the water so this didn’t get too wet.
The area under the sofa stayed dry, presumably because the valve was open so there wasn’t any pressure on that valve.
Could that drain valve for the fresh water tank be any harder to get to? The prior owner apparently tied a string to the valve pull and ran it up behind the sofa to make it easier to open, but closing it requires some acrobatics. I will have to pull the sofa out to service and reseal the valves if needed.