Well I though they have a one way valve in them? How long is your feeder line? Like my Tornado is on the side of the black tank and I have about three foot of line to relocate the hose fitting where I can get to it easy.
I would think it would only be what is left behind the one way valve would drain out but that should drain in 20 seconds. Perhaps if your feeder line is higher than the flush nozzel and it will take longer to drain?
2017 Axis / 25.4: Big Foot Levelers, Aims 1200 Watt inverter, Ceramic Window Tint, Full River AGM 6V batteries, Front Hellwig Sway bar / Rear Track Arm, Safe T Plus Steering Stabilizer, PD 4655 converter, Hard Wired Surge Guard, WEboost + WiFi Ranger, LED Headlights/Driving lights, TPMS, Surge tank, Tornado flush, Viair Compressor