Only time any water should be drawn from the on board fresh water tank is when the water pump is on.
If on city water and water pump off - should not be any draw.
I have used the water pump/fresh water tank to boost pressure for some poor campground supplies.
I've seen many reports of fresh water tanks filling when on city water (bad check valve or debris in it) - but haven't seen the 'draining' problem.
Not yet retired...
Florida (Michigan transplant)
2014 Hurricane 32A
2000 Infinity (previous)