<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/ObjectManager/etc/config.xsd">
    <preference for="MagentoCheckoutModelDefaultConfigProvider" type="TestSalesModelCheckoutDefaultConfigProvider" />
</config>

<?php
namespace TestSalesModelCheckout;

class DefaultConfigProvider extends MagentoCheckoutModelDefaultConfigProvider
{

    private function getTotalsData()
    {
        die('override');
       // Override stuff
    }

}
?>

class DefaultConfigProvider extends MagentoCheckoutModelDefaultConfigProvider
{

class DefaultConfigProvider extends MagentoCheckoutModelDefaultConfigProvider
{

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="MagentoCheckoutModelDefaultConfigProvider">
        <plugin name="custom_defaultconfigprovider" type="TestSalesPluginDefaultConfigProvider" />
    </type>
</config>

<?php
namespace TestSalesPlugin;

class DefaultConfigProvider
{
    public function afterGetConfig($subject, $result)
    {
        if (isset($result['totalsData']) && $result['totalsData']) {
            $custom = // add your custom here;
            $result['totalsData'] = $custom;
        }
        return $result;
    }
}