Eighteen feet of little bitty pipe with a lot of bends = a lot of back pressure.
I would re route it so you only have about 2 - 3 feet of exhaust pipe.
If you are really good, you could use bigger pipe.
Mike in Colorado
2004 Fleetwood Pace Arrow 37c,(Jezebel) Ultra RV ECM / TCM, Bilstein's, Sailun's & Sumo's all round, pushed by a 2014 Chrysler T&C.