How to create additional full text RSS feeds

Here is how to create a custom feed:

first create a new file in your theme’s directory , name it your-custom-feed.php and put this code inside

<?php

     /**
      * custom RSS feed.
      *
      * @package WordPress
      */

     header('Content-Type: ' . feed_content_type('rss-http') . '; charset=" . get_option("blog_charset'), true);
     $more = 1;

    echo '<?xml version="1.0" encoding="'.get_option('blog_charset').'"?'.'>'; ?>

    <rss version="2.0"
        xmlns:content="http://purl.org/rss/1.0/modules/content/"
        xmlns:wfw="http://wellformedweb.org/CommentAPI/"
        xmlns:dc="http://purl.org/dc/elements/1.1/"
        xmlns:atom="http://www.w3.org/2005/Atom"
        xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
        xmlns:slash="http://purl.org/rss/1.0/modules/slash/"
        <?php do_action('rss2_ns'); ?>
    >

        <channel>
            <title><?php bloginfo_rss('name'); wp_title_rss(); ?></title>
            <atom:link href="https://wordpress.stackexchange.com/questions/22849/<?php self_link(); ?>" rel="self" type="application/rss+xml" />
            <link><?php bloginfo_rss('url') ?></link>
            <description><?php bloginfo_rss("description") ?></description>
            <lastBuildDate><?php echo mysql2date('D, d M Y H:i:s +0000', get_lastpostmodified('GMT'), false); ?></lastBuildDate>
            <language><?php echo get_option('rss_language'); ?></language>
            <sy:updatePeriod><?php echo apply_filters( 'rss_update_period', 'hourly' ); ?></sy:updatePeriod>
            <sy:updateFrequency><?php echo apply_filters( 'rss_update_frequency', '1' ); ?></sy:updateFrequency>
            <?php do_action('rss2_head'); ?>
            <?php while( have_posts()) : the_post(); ?>
            <item>
                <title><?php the_title_rss() ?></title>
                <link><?php the_permalink_rss() ?></link>
                <comments><?php comments_link_feed(); ?></comments>
                <pubDate><?php echo mysql2date('D, d M Y H:i:s +0000', get_post_time('Y-m-d H:i:s', true), false); ?></pubDate>
                <dc:creator><?php the_author() ?></dc:creator>
                <?php the_category_rss('rss2') ?>

                <guid isPermaLink="false"><?php the_guid(); ?></guid>
                <description><![CDATA[<?php the_excerpt_rss() ?>]]></description>
            <?php if ( strlen( $post->post_content ) > 0 ) : ?>
                <content:encoded><![CDATA[<?php the_content_feed('rss2') ?>]]></content:encoded>
            <?php else : ?>
                <content:encoded><![CDATA[<?php the_excerpt_rss() ?>]]></content:encoded>
            <?php endif; ?>

                <wfw:commentRss><?php echo esc_url( get_post_comments_feed_link(null, 'rss2') ); ?></wfw:commentRss>
                <slash:comments><?php echo get_comments_number(); ?></slash:comments>
        <?php rss_enclosure(); ?>
            <?php do_action('rss2_item'); ?>
            </item>
            <?php endwhile; ?>
        </channel>
    </rss>

then add a simple function to call that template file using do_feed_$hook

//load feed template
function create_my_customfeed() {
    load_template( TEMPLATEPATH . 'your-custom-feed.php'); 
}
add_action('do_feed_mycustomfeed', 'create_my_customfeed', 10, 1);

Now when you access http://yoursite.com/?feed=mycustomfeed you will get a full text feed, no mater what you define inside WordPress admin.

Bonus

if you want to create a rewrite rule for your custom feed so your url could be:

http://yoursite.com/mycustomfeed.xml
as well as a
http://yoursite.com/feed/mycustomfeed/

add this code to your functions.php file:

//create feed rewirte rule

function custom_feed_rewrite($wp_rewrite) {
    $feed_rules = array('feed/(.+)' => 'index.php?feed=' . $wp_rewrite->preg_index(1),'(.+).xml' => 'index.php?feed='. $wp_rewrite->preg_index(1));
    $wp_rewrite->rules = $feed_rules + $wp_rewrite->rules;
}