Can you run a hose up the dump drain? If so, you can buy a hose end attachment at CW used to flush black tanks from the toilet side. Just attach it to a hose and feed it into the grey tank. turn on the water and slide it around for a while.
Also, try filling the grey tank half full with hot water, Dawn and drive around a while. And if you leave the grey/galley tank open when hooked to full hookups, stop doing that.
Grey water will backup into the sink or shower when over filled. Won't burst.
Tom Wilds
Blythewood, SC
2016 Newmar Baystar Sport 3004
2015 Jeep Wrangler 2dr HT