We had somewjhat the same problem. The black would flow no problem grey on the other hand could take up to 45 mins to drain.
It was still under warranty so we had Monaco look at it. When they orginally installed the tank they left bits and pieces of the wood in the tank when they drilled to connect it. They did fix it, 45 man hoyrs later. Works okay now, a touch slow but can't compalin. I also had a mercerater put on and that seems to have helped also. Being level or out of level also has a play in it too.