I'm having a significant loss of power when pulling my 5th wheel toy hauler. Particularly going up steep grades- pulls down considerably. Have already changed fuel filter, cleaned exhaust back pressure sensor tube and replaced sensor, new air cleaner. None of these changes made a difference. Truck is automatic, shifts fine, cranks up fine- no error codes lighting up on dash, no smoke. Has anyone else had this issue and how to fix? What else to check?