Cool idea.
There's a related video here:
https://rumble.com/v4evl1-fill-er-up.html that shows it works just like you'd think.
Any toxicity concerns? I don't drink the water in my fresh tank but lots of folks do.
I'm surprised that there isn't a smaller version of a fuel nozzle like device specifically made for filling water jugs or similar items.
On my van's gravity fill there is a distinct change in the sound as the water rises up the filler tube so you know when it is getting close but some type of auto shut off filler would be useful.