Being the age of your Outback, yours may not have a valve that you can flip to winterize. You may have to disconnect the "in" water line from the fresh water tank to the water pump, disconnect at the pump, and attach a hose, with the correct connector to suck up the anti-freeze from the jug.
All 3 of my first travel trailers were like this. However, on my last TT, (Outback), I installed a flip valve at the water pump that would flip from the fresh water tank (in), to a hose that was put in the gallon anti-freeze jug. You can do the same thing.
Keystone Springdale:
Keystone Outback:
The arrows are pointing to the filter. The self-installed switch is on the top of the pump ("in" line).