You may have built up some internal pressure if the overflow can't let the water out as fast as you put it in. I have read about some folks that fill their tanks slowly to assure they don't "overfill". That said, I have never had that problem. I just turn off the water as soon as I see overflow happening. As for siphoning, it usually stops as soon as the water level drops below the vent.
As to the indicator light: The indicator might have a broken wire not connecting the sensors. These light units are notoriously poor. I can see my water level by looking in the water pump compartment.
Life is a journey, not a destination.
2005 Georgetown 359ts
Bill & Virginia, Kentucky
Nights camped in 2011...78 , 2012...73,
Nights camped in 2013...123, 2014 ...101