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The RV antifreeze hose is the one with a valve on it, that isn't connected to anything. Usually, it's close to the pump. That is the one that gets inserted into the bottle of RV antifreeze. The valve must be open for it to be drawn into the pump.
Below is what my pump and valves look like. You can see about two inches of pink antifreeze in the clear vinyl hose. The valve the hose is connected to is off as I'm not pumping any antifreeze at this time. If you follow the pipe from the valve down to the tee, the pipe leading from the right right side of the tee is the one that goes to my fresh water tank. The valve for it is on the vertical pipe. It's currently in the open position, but needs to be closed when pumping the antifreeze.
There are many reasons why you might not be getting suction, but leaving that valve open is a common one. If left open, all you'll suck is air, assuming the tank's empty, which the tank should be if you're winterizing. The tank will drain no matter what position the valve is in so it's often overlooked.