WordPress: Use Custom Template From Plugins

gistfile1.php

/**
 * Route single page to custom template
 * 
 * @return string of path
 */
function fr_route_template( $single_template ){
	
	// Put any conditional tag here. This one assumes you want to serve custom template for CPT titled "newsletter"'s single page
	if( is_singular( 'newsletter' ) ){

		// Get template path, assuming the structure is /wp-content/your-plugin/templates/single.php
		$template_path = plugin_dir_path( __FILE__ ) . 'templates/single.php';

		return $template_path;

	} else {

		return $single_template;
	}
}
add_filter( 'template_include', 'fr_route_template' );