Is there an external water management bay on the outside of the coach somewhere, where you would attach the shore water hose? MIne is shared with my shore power cable storage at the driver's side rear of the coach. In my Sunstar and in my Navion, there are 2 smaller handles which are the controls for the line drains. If I put them in the drain position and open up the water pressure, I'll see what you're seeing too.
They're oval shaped controls, metallic color. Try turning one or both of them to see if the drains stop draining.
2018 (2017 Sprinter Cab Chassis) Navion24V + 2016 JKU (sold @ ????)
2016 Sunstar 26HE, V10, 3V, 6 Speed (sold @ 4600 miles)
2002 Roadtrek C190P (sold @ 315,000kms)