Assuming you have a Norcold refrigerator,
Why winterize the ice maker? It is designed to freeze. Shut off the incoming water at the under-counter filter, remove both incoming and outgoing water lines from the ice maker solenoid and drain the valve body, the line to the ice maker automatically drains. The incoming line from the undercounter filter may be cleared with regulated air pressure then turn the flow valve back off on the filter housing.
I've been doing that since 2013 without once adding RV antifreeze into the ice maker system or associated plumbing.
This may be useful: http://bryantrv.com/docs2/docs/icemaker.pdf
If you have a residential refrigerator the above will not apply.