Marbill,
We also have the 3055RL. The valve is definitely behind the removable panels in the basement.
First remove the panel closest to the UDC. There are 3 screws that go through the UDC (from the outside) and into the panel. There are 3 screws on the DS of the panel. With these 6 screws out, you can remove the panel and stick your head/shoulders in there.
My ice maker valve was just below the floor (roof as you are in basement) and right behind the UDC enclosure.
A real b***h
to get to. But as Ray said , there is a 1/4 turn valve and the clear tubing.
Good luck.
PS while in there, you ought to check all the other water connections. I found my fresh water tank vent lying on the floor disconnected. Fortunately, not a big deal, but it could have been something else.