You shouldn't be pouring the antifreeze into your fresh water tank to begin with. You need to start by installing a "winterizing bypass kit" on your water pump if you don't already have one; that is what you use to "pump" the antifreeze directly from the jug into your lines.
The answer to your question is, your drain is probably not draining the tank completely (not at all unusual). This will dilute your antifreeze and you will probably need about 10 gallons of antifreeze by trying to pump antifreeze out of your fresh water tank instead of pumping it from the jug, which would take about 2 gal.
And secondly, you will play hell trying to get that antifreeze out of your fresh water tank in the spring. Be prepared to fill and drain that tank about 10 times.