On my rig, if the fresh water pump is turned off, no water leaves the fresh water tank whether hooked up to shore water or not.
I can hear my water pump when it operates. Normally, if shore water is on and the fresh water pump is on, my water pump will not operate.
You must have a three way switch with one inside the coach and one outside the coach?
If so, maybe the indicator light on the outside switch is reversed?
With no shore water on, the outside switch "on" and a water faucet open, does water flow from the tank? Does it then stop when the switch is turned off?
If not, you could have a wiring problem.
If it does, then a leak at the tank would seem to be the only other likely cause of water loss.
1995 CC Magna #5280
C8.3L 300hp Cummins, 31,000lbs