the_content()
(and many others) must be inside The Loop.
For retrieving a post/page/custom-post-type data outside the Loop, take a look here:
Get post content from outside the loop
The working code, based on your example, would be:
<div class="middle-inner">
<div class="center-inner">
<!-- This is content -->
<?php
global $post;
query_posts("post_type=roomtype&orderby=name");
if ( have_posts() ) {
?>
<div class="title">
<p class="h1"><?php _e ('Rooms', 'hotel');?></p>
</div>
<?php $i=0; while ( have_posts() ) { the_post(); $i++;
//$thumbnail_id = get_post_thumbnail_id($post->ID);
// $link_image = wp_get_attachment_image_src( $thumbnail_id, 'roomtype-image' );
//$link_thumbnail = wp_get_attachment_image_src($thumbnail_id);
<?php echo 'I NEED MY PAGE CONTENT HERE - just add the following line' ?>
<?php the_content(); ?>
if(has_post_thumbnail()) {
$thumbnail_id = get_post_thumbnail_id($post->ID);
$link_image = wp_get_attachment_image_src( $thumbnail_id, 'roomtype-image' );
} else {
$args = array(
'post_type' => 'attachment',
'numberposts' => -1,
'post_parent' => $post->ID,
'post_mime_type' => 'image'
);
$attachments = get_posts($args);
$link_image = wp_get_attachment_image_src( $attachments[0]->ID, 'roomtype-image' );
}
?>