Create a page, load a template file from plugin, pass input variables from one template to another via query_var

unique-plugin_theme_files_custom-search.php

<?php
/**
 * The template for displaying search results pages.
 *
 * @package WordPress
 * @subpackage Twenty_Fifteen
 * @since Twenty Fifteen 1.0
 */

get_header(); ?>

<section id="primary" class="content-area">
    <main id="main" class="site-main" role="main">
        <?php  
        /**
         * Here I am getting the zip code from the input on test-page.php
         */
        $zip = get_query_var( 'dealer-search-form', 1 );
        var_dump($zip);
        ?>
        <?php if ( have_posts() ) : ?>

            <header class="page-header">
                <h1 class="page-title"><?php printf( __( 'Search Results for: %s', 'twentyfifteen' ), get_search_query() ); ?></h1>
            </header><!-- .page-header -->

            <?php
            // Start the loop.
            while ( have_posts() ) : the_post(); ?>

                <?php
                /*
                 * Run the loop for the search to output the results.
                 * If you want to overload this in a child theme then include a file
                 * called content-search.php and that will be used instead.
                 */
                get_template_part( 'content', 'search' );

                // End the loop.
            endwhile;

            // Previous/next page navigation.
            the_posts_pagination( array(
                'prev_text'          => __( 'Previous page', 'twentyfifteen' ),
                'next_text'          => __( 'Next page', 'twentyfifteen' ),
                'before_page_number' => '<span class="meta-nav screen-reader-text">' . __( 'Page', 'twentyfifteen' ) . ' </span>',
            ) );

        // If no content, include the "No posts found" template.
        else :
            get_template_part( 'content', 'none' );

        endif;
        ?>

    </main><!-- .site-main -->
</section><!-- .content-area -->

<?php get_footer(); ?>

unique-plugin_theme_files_test-page.php

<?php
/**
 * The template for displaying pages
 *
 * This is the template that displays all pages by default.
 * Please note that this is the WordPress construct of pages and that
 * other "pages" on your WordPress site will use a different template.
 *
 * @package WordPress
 * @subpackage Twenty_Fifteen
 * @since Twenty Fifteen 1.0
 */

get_header(); ?>

	<div id="primary" class="content-area">
		<main id="main" class="site-main" role="main">
			<form action="<?php echo esc_url( home_url("/custom-search/")); ?>" id="searchform-dealer" method="POST">
				<div>
					<input type="text" id="dealer-search-form" name="dealer-search-form" value="" placeholder='Enter zip code...' />
					<input type="submit" value="GO!" id="searchsubmit" class="button" />
				</div>
			</form>
		</main><!-- .site-main -->
	</div><!-- .content-area -->

<?php get_footer(); ?>

unique-plugin_unique-plugin.php

<?php
/*
Plugin Name: unique-plugin
Description: this is a description!
*/

/* include custom meta boxes as needed */

define( 'MI_PATH', plugin_dir_path(__FILE__) );
define('MI_URL', plugin_dir_url(__FILE__));

function create_pages(){
    $user_id = get_current_user_id();
    if (!post_exists('Custom Search')){
        $defaults = array(
            'post_title'            => 'Custom Search',
            'post_content'          => '',
            'post_name'             => 'custom-search',
            'post_status'           => 'publish',
            'post_type'             => 'page',
            'post_author'           => $user_id,
            'ping_status'           => 'closed',
            'post_parent'           => 0,
            'post_excerpt'          => ''
        );
        $post_id = wp_insert_post( $defaults );
    }
    if (!post_exists('Test Page')){
        $defaults = array(
            'post_title'            => 'Test Page',
            'post_content'          => '',
            'post_name'             => 'test-page',
            'post_status'           => 'publish',
            'post_type'             => 'page',
            'post_author'           => $user_id,
            'ping_status'           => 'closed',
            'post_parent'           => 0,
            'post_excerpt'          => ''
        );
        $post_id = wp_insert_post( $defaults );
    }
}

add_action('admin_init','create_pages');

function include_template_files() {
    global $wp;

    if (is_page('custom-search')) {
        $templatefilename = 'custom-search.php';
        $template = MI_PATH . '/theme_files/' . $templatefilename;
        include($template);
        exit;
    }
    if (is_page('test-page')) {
        $templatefilename = 'test-page.php';
        $template = MI_PATH . '/theme_files/' . $templatefilename;
        include($template);
        exit;
    }

}

add_action('template_redirect', 'include_template_files');

function add_referring_pic_data_query( $vars ){
    $vars[] = 'dealer-search-form';
    return $vars;
}
add_filter( 'query_vars', 'add_referring_pic_data_query' );