Well... I don't KNOW what happened, but I can guess...
The fresh water system is designed to take water either from the city water inlet or from the tank, via the water pump. Those two sources have to connect somewhere before entering the main MH plumbing.
To avoid the problem you seem to have, two back flow preventers are needed - one on each of the two input lines. I don't know where these are - perhaps one is built in to the water pump? - but the one on the tank/water pump line is at least partially open, allowing the city water to flow backwards through the pump and into the tank.
Since water isn't gushing out of the gravity fill, I'd guess that the opening is fairly small - maybe something is caught it in, preventing it from closing completely. One thought - you might run the pump, forcing water through that path, which might clear an obstruction.
Anyway, all of this is speculation on my part....