Fresh Water System Cleaning and Maintenance
Its the flow dynamics of a cubcial tank.....
The flow characteristics through a cubical tank will leave ''dead zones'' that will take zillions of gallons of flush water to remove/dilute any residual. Most flat bottomed tanks & water heaters will not totally empty when mounted level ... always leaving a little bit of residual no matter how well you think you drain them (only way is to remove & rotate them .... too much problem on a boat).
Do an ''alliquot'' (a mathematical ''trick'' in dilution technique) washing to QUICKLY dilute the residual anti-freeze: fill tank with a few gallons, remove lowest freshwater pipe connection/check-valve and let totally drain (or pump DRY), re-fill with a few gallons into the tank, remove etc.... do several/many times. This will rapidly dilute (mathematically called: exponential decay dilution) the antifreeze that remains.