Cannot print $variables using template_preprocess_views_view_rss() in readable format
My solution was to use kint() and print_r() with array_keys() to locate all the objects for manipulation.  In some cases I was debugging with the kint() display and in others just looking at plain text by using the other mentioned functions.

For note the template_views_view_rss() and template_views_view_row_rss() hooks are located in view.theme.inc (core) and no where in documentation on drupal.org for D8.  I believe they are soon to be deprecated.

// Specific feed filter
if( $variables['view']-&gt;id() == 'blog' &amp;&amp; $variables['view']-&gt;current_display == 'feed_1') {
  // Iterate rows
  foreach($variables['rows'] as $key_1 =&gt; $row) {
    // Do stuff
    foreach($row['#row']-&gt;elements as $key_2 =&gt;  $element) {
      // Do stuff
    }
  }
}