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Some people do pour antifreeze into the fresh water holding tank but the best way to suck the antifreeze through your system is to remove the suction hose from the water pump and replace with a hose that you can put directly into a container of antifreeze. You do not have to worry about the fresh water holding tank freezing, as long as you have completely drained it and left the drain valve open.
Very often that is where it is. The easiest is to find the city water connection from the exterior of the unit. The pump is located close to that, only it is usually accessible from the interior of the unit. You can also turn it on with a tap open and listen for it running.