I just bought a 2017 Sierra 371REBH. I am currently using the water holding tank. Mine has a tank vent in the action area and I have noticed if I just turn the water on to fill the holding tank, it sprays water out as if it builds up an air bubble behind a small amount of water that gets in the vent tube. I discovered that if I blow air into the vent when I turn the fill hose on, It doesn't blow water out. You can also listen and if there is water in the vent it will chug. If you blow air into the vent, it doesn't chug but you can hear the water filling freely. Hope this helps!
2016 Chevrolet 3500HD
2017 Sierra 371REBH