When I first purchased my RV there was so much congealed grease on the floor of the grey water tank that it effectively sealed an 18" long crack located in the tank floor. That is it sealed it until I tried to actually store grey water in the tank rather than just letting it pass through to the drain. It didn't take much water pressure to breach the congealed grease seal and start the tank leaking. I've since replaced the tank.
Anyone who wants to dump grease down their drain, go ahead your plumber will appreciate the future work.
Steve