OP, you need to fix your signature, so we know what kind of unit you have, and please list all the mod's too. See mine below.
If the pipe has a cap on it, it could be the sink vent. but with no picture, you know we're just guessing.
Mike in Colorado
2004 Fleetwood Pace Arrow 37c, 8.1 gasser, (Jezebel) Ultra RV ECM / TCM and rear track bar, PPE deep Tx pan, Bilstein's, Sailun's & Sumo's all round, pushed by a 2002 Grand Caravan, on a Master Tow Dolly.