How to get CiviCRM Views to work in Drupal 8 with separate CiviCRM database
I had the same problem today, so I looked in the source code.

In civicrm.install there's a function called _civicrm_get_db_config.  It's looking in the $databases array for an element with a key civicrm.  So I added this to settings.php:

$databases['civicrm']['default'] = array (
  'database' =&gt; 'civi_db_name',
  'username' =&gt; 'mysqluser',
  'password' =&gt; 'redacted',
  'prefix' =&gt; '',
  'host' =&gt; 'localhost',
  'port' =&gt; '3306',
  'namespace' =&gt; 'Drupal\\Core\\Database\\Driver\\mysql',
  'driver' =&gt; 'mysql',
);


I then had to clear all caches before it worked, but work it does.