One solution would be to plumb the output to a seldom used sink or tap. A outdoor tap would allow you to water the garden when changing your stored supply.

I wouldn't worry too much about contaminated central supply if the system you are drawing from is reliable, as most are. What are the odds any major water supply fault would occur on the once a week average use pattern when you just happen to use that sink. Or, even rarer, if you were changing out the stored water on a schedule, when you were making the change.

There are also some central supply faults that can be corrected. If it is bacterial you could cut off supply to depressurize the tanks and manually add bleach.

Stored water, if clean to start with and kept away from light stored pretty much indefinitely. You don't really need to change out the stored water very often.