Read comment below for how to use.

functions.php

<?php

/**
 * Get media of perticular topic or forum
 *
 * @param int $item_id Id of forum
 * @return array       Array of media ids associated with the forum
 */
function rtcamp_get_perticular_forum_media( $item_id ) {

	if ( empty( $item_id ) ) {
		return;
	}

	$medias  = array();
	// Get the topics of forum.
	$topics = bbpress_get_data( $item_id, 'topic' );
	if ( ! empty( $topics ) && is_array( $topics ) ) {
		foreach( $topics as $topic_id ) {
			// Get the replys of topic
			$replys = bbpress_get_data( $topic_id, 'reply' );
			if ( ! empty( $topics ) && is_array( $topics ) ) {
				foreach( $replys as $reply ) {
					// Get medias attached to topic.
					$medias[] = rtmedia_get_medias( $reply );
				}
			}
		}
	}
	if ( ! empty( $medias ) ) {
		$medias = array_values( array_filter( $medias ) );
	}
	return $medias;
}

/**
 * Delete media of perticular topic or forum
 *
 * @param int $item_id Id of forum
 * @return array       Array of media ids associated with the forum
 */
function delete_media_of_forum( $item_id ) {

	if ( empty( $item_id ) ) {
		return;
	}

	// Get medis from forum id.
	$medias = rtcamp_get_perticular_forum_media( $item_id );

	if ( ! empty( $medias ) ) {
		global $wpdb;

		foreach ( $medias as $media_id ) {

			$where = array(
				'media_id' => $media_id
			);

			// Delete attachment.
			wp_delete_attachment( $media_id );

			// Delete entry from rtmedia table.
			return $wpdb->delete( $wpdb->prefix . 'rt_rtm_media' , $where );
		}

	}

}

// Get the data.
function bbpress_get_data( $item_id, $context ) {
	$args = array(
		'no_found_rows' => true,
		'fields' => 'ids',
		'posts_per_page' => 500,
		'post_parent' => $item_id,
		'post_status' => 'publish',
		'post_type' => $context
	);
	$data = new WP_Query( $args );
	return $data->posts;
}

// Get medias by reply.
function rtmedia_get_medias( $id ) {
	$model = new RTMediaModel();
	$media = $model->get_media( array(
		'context_id' => $id,
	), false, false );
	if ( empty( $media ) ) {
		return;
	}
	return $media[0]->media_id;
}

If you want to fetch the media associated with particular forum the use following function and pass forum ID as an argument

rtcamp_get_perticular_forum_media( $item_id )

If you want to fetch and delete those media, then use the following function and pass forum ID as an argument

delete_media_of_forum( $item_id )