"Poo-Piles" are difficult to remove once they have had a chance to establish themselves... caused by not using/keeping enough water in the tank, especially after you dump... and can cause lingering smells. If you have a straight pipe into your BW tank you can check using a flashlight. Often times they can be broken up using hot water and a long stick. Dumping several bags of ice down the toilet after dumping and before hitting the road can help with cleaning the tank. But chances are it's a blocked vent pipe as others have suggested.