OK - good question. I have a 2012 Rockwood travel trailer. So, now it might be helpful for us to know which model you have. For example mine is a Windjammer 3008, equipped with 3 drains. One is for the galley (the kitchen sink), another for the "grey water" which is for the bathroom sink and shower, and black water (the toilet).
Did your problem just start? Are the grey and black valves joined together, such as do they both exit out the same pipe.
Not knowing more at this point it is hard to decide the best solution, however I would close both black and grey valves at this time and add calgon water softener to both the toilet and the grey water tank/s. Let that stay in them until near full then open the drains one at a time to see how they each drain. Then repeat the process.
You should see more replies soon.
Best of luck.
2012 Rockwood Windjammer 3008 W
2007 Tundra Reg Cab 4X4 SR5 5.7L