Telling us the year of your 3780 would help. I believe you are referring to the fresh water tank vent. Water flowing out indicated the tank is full. There is a valve in the convenience center that directs the incoming water to either the coach or to fill the fresh water tank.
If you are hooked to an external l water supply and the tank if filling there is likely only two reasons. The valve is set to fill the tank (switch it coach supply where it should be) or the valve has failed (frozen and cracked?).
If you are running off the onboard water pump (which was just replaced?) then I am not sure what is going on. Possibly reversed water line connections? But where would the pump get water to push back into the tanks? Very strange.
Bob Rohrmann has previously posted a diagram of the water system in Montanas. Perhaps a Search will uncover it.
On Edit: I found Bob's diagram. Look for Post #3 in this Thread:
https://www.montanaowners.com/forums...ad.php?t=85772