Issue with multiple orderby values

In your meta query you also need explicitly query posts that don’t have that meta key set. You can do this by nesting meta queries like so:

'orderby'    => array(
    'meta_value_num' => 'ASC', 
    'menu_order'     => 'ASC'
), 
'meta_query' => array(
    'relation' => 'AND',
    array(
        'relation' => 'OR',
        array(
            'key'  => 'newprice',
            'type' => 'decimal',
        ),
        array(
            'key'     => 'newprice',
            'value'   => '',
            'compare' => 'NOT EXISTS',
        ),
    ),
    array(
        'key'     => 'datum',
        'compare' => '>=',
        'value'   => strtotime('-1days'),
    ),
),

Also note that you need to set 'meta_key' on the query so that when you orderby 'meta_value_num' it orders by the right meta key.

Actually, it doesn’t seem to be necessary if you have a meta_query, and excluding it will save you a join.