City water can not siphon water from fresh water tank
Water leaves a fresh water tank via pump suction, siphoning out vent, siphoning out overflow or thru tank drain
Water goes into tank via gravity fill port OR a 'fill' valve
If in city water the 'fill' line would be flowing water into tank if fill valve was cracked open or leaking thru as it has city water pressure against it
Pump ON and city water pressure low enough that pump runs using up water in tank
Siphoning out vent, overflow
Tank drain leak
And what is this about having to unpack & dry out basement compartment ---twice a year :H LEAKS!