Custom Menu Variables

Does anyone know where the info about the post/page/CPT is passed into the menus. It seems the menu uses a whole different set of variables and getting any post/page information is next to impossible… any help?

For example:
$item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;

title is the item title, rather than the actual data I need.

Would love to use info like excerpt, thumbnail etc in the menus.