If you set the xplicitArray option to false, when it parses it won't create those arrays where you didn't expect. Like this:

function parseBody(body) {
    var parseString = require('xml2js').parseString;

    const options = {
        explicitArray: false
    };

    var xml = body
    parseString(xml, options, function (err, result) {
        console.log("cust fname: " + result['soapenv:Envelope']['soapenv:Body']['int:readResponse']['response']['cust']['fName']);
    })
}
var input = '<soapenv:Envelope xmlns:soapenv=\"http://schemas.xmlsoap.org/soap/envelope/\" xmlns:int=\"realURLHere"><soapenv:Header/><soapenv:Body><int:readResponse><response><request>?</request><cust><fName>?</fName></cust></response></int:readResponse></soapenv:Body></soapenv:Envelope>';
parseBody(input);

Furthermore if you remove the prefixes it can get even cleaner:

function parseBody(body) {
    var parseString = require('xml2js').parseString;
    var stripNS = require('xml2js').processors.stripPrefix;

    const options = {
        tagNameProcessors: [stripNS],
        explicitArray: false
    };

    var xml = body
    parseString(xml, options, function (err, result) {
        console.log("cust fname: " + result.Envelope.Body.readResponse.response.cust.fName);
    })
}
var input = '<soapenv:Envelope xmlns:soapenv=\"http://schemas.xmlsoap.org/soap/envelope/\" xmlns:int=\"realURLHere"><soapenv:Header/><soapenv:Body><int:readResponse><response><request>?</request><cust><fName>?</fName></cust></response></int:readResponse></soapenv:Body></soapenv:Envelope>';
parseBody(input);```