How about a three way valve on the fresh water tank to water pump (suction side). And a three way valve on output side of your water pump.
Put another suction line on the three way valve and run that out to your water jug.
Put another pressure side line off the three way valve on the pressure side of pump and route that down into your fresh water tank. Should be able to see where the current fill line goes down into your tank. Heck, you could actually use a T into the existing fresh water fill line or something like that. That way you can't overfill.
Working all this out on the fly, someone correct me if I'm wrong on my thinking. :-)
Doh! on second thought, use the existing antifreeze fill line and then add three way valve on output side of pump. Plumb the new output line as I mentioned above.