zman....Without looking at mine, I think the two positions are "City Water" and "Fill". Two things.....when you say it fills your fresh tank are you talking about a matter of minutes or days. If it's minutes, the labeling/connection to the valve may be backwards. If it's taking days, the valve probably has a slow leak.
I'm not sure if it's possible, it could also be a leak in the water pump seal/valving and it's allowing city walter to back flow through the pump and back into the tank. Good Luck
Don & Mary
2019 Newmar Dutch Star 4018 (Freightliner)
2019 Ford Raptor