I Had a Journey that I had to be very careful about filling and emptying the black tank. If you have the same situation as I here is what I found. The commode was on the passenger side of the coach. Of course the discharge side was on the drivers side. The commode dumped straight down into the tank but because of the location the tank was very shallow and I believe sloped to the drivers side and drain. This made using a considerable about of water necessary to make sure everything "floated" to the water in a near empty tank rather than just stick to the dry bottom. Also I would not empty the tank until nearly full. And many times would travel with a fair amount in the black tank so I could get the benefit of it sloshing around to keep from getting the dreaded pile.