We have the same problem with gray water coming up into the shower when under way. The drain valve has threads to screw into the shower drain, but the shower drain is missing the threads. We have an appt with Rease to have this repaired. Not sure if this is your problem. We always try and travel with as empty a gray water tank as possible. But life is never perfect...!
Carla Jean Pugh (and my guy, Ned)
"Pepé Le Pugh"
2016 Escape 21'
2005 Toyota 4Runner