Problems with Dumping Black and Grey tanks

[nekraus47]

We own a 2010 Montana 3150 RL which we purchased new in the fall of 2009. We have had a problem with the back and grey tanks emptying completely since it has been new. One time it will dump as it should and the next time it runs very slow and will not empty all the way. We have had it back to our dealer twice while under warranty and both time they tell us that they can find nothing wrong, yet the problem continues to happen. Once in awhile it will dump as it should. We have contacted Keystone and they have no solutions. They tell us that they have not had anyone else contact them with this problem. Both the dealer and Keystone tell us that the tank has to be at least 2/3 full for it to dump completely, so we make sure that the gauges show at least 2/3 full, but that does not help either. This past week we went to dump, again it ran slow and the gauge showed 1/3 full so we pulled away from the dump station, turned the trailer around and dumped from the other side of the dump station. It then emptied completely. We even had our local plumber go thru the system with a camera to see if there were any obstructions. He did not find any. We are tired of fighting this, and wonder if any one else out there have this problem. Any ideas would be appreciated.