Inserting custom fields into single.php

Yes, there is ‘the_content’ filter. See the codex.

For instance:

add_filter( 'the_content', 'my_the_content_filter', 20 );

/**
 * Add a test to the end of every post page.
 */
function my_the_content_filter( $content ) {

    if ( is_singular('hotels') ){
        //If a single post is being viewed, add something
        //Or get the value of a custom field and append it
        $post_id = get_the_ID();
        $custom_field_value = get_post_meta($post_id, 'Address', true);
        $content .= $custom_field_value;
    }
    // Make sure you returns the content, edited or otherwise
    return $content;
}