I don't know how you're parsing the xml, what I can see is your mapping class does not seem to be in according with your swagger, try with this class
using System;
using System.Xml.Serialization;
using System.Collections.Generic;
namespace Xml2CSharp
{
[XmlRoot(ElementName="item")]
public class Item {
[XmlElement(ElementName="BUKRS")]
public string BUKRS { get; set; }
[XmlElement(ElementName="WERKS")]
public string WERKS { get; set; }
[XmlElement(ElementName="NAME1")]
public string NAME1 { get; set; }
[XmlElement(ElementName="REGIO")]
public string REGIO { get; set; }
[XmlElement(ElementName="MATKL")]
public string MATKL { get; set; }
[XmlElement(ElementName="MATNR")]
public string MATNR { get; set; }
[XmlElement(ElementName="LGORT")]
public string LGORT { get; set; }
[XmlElement(ElementName="GRDIP")]
public string GRDIP { get; set; }
[XmlElement(ElementName="TRNDATE")]
public string TRNDATE { get; set; }
[XmlElement(ElementName="VOL_NATURAL")]
public string VOL_NATURAL { get; set; }
[XmlElement(ElementName="GRDIP_RUN")]
public string GRDIP_RUN { get; set; }
[XmlElement(ElementName="VOL_RUNNING")]
public string VOL_RUNNING { get; set; }
}
[XmlRoot(ElementName="T_OUTPUT")]
public class T_OUTPUT {
[XmlElement(ElementName="item")]
public List<Item> Item { get; set; }
}
[XmlRoot(ElementName="YV_PIPELINE_PLANT_DIP_DETAILS.Response", Namespace="urn:sap-com:document:sap:rfc:functions")]
public class YV_PIPELINE_PLANT_DIP_DETAILS.Response {
[XmlElement(ElementName="E_MSG")]
public string E_MSG { get; set; }
[XmlElement(ElementName="T_OUTPUT")]
public T_OUTPUT T_OUTPUT { get; set; }
[XmlAttribute(AttributeName="ns0", Namespace="http://www.w3.org/2000/xmlns/")]
public string Ns0 { get; set; }
}
}