Drupal Rooms Show Booking Unit Name in Confirmation Email
You could create a custom token to pull that info in. 
Hook_token_info is the hook that makes a token available.  

https://api.drupal.org/api/drupal/modules%21system%21system.api.php/function/hook_token_info/7

While hook_token is the hook that actually replaces it.

https://api.drupal.org/api/drupal/modules%21system%21system.api.php/function/hook_tokens/7

Since it's a multivalue field, you may need to include a numeric operator on the end (e.g. node_room_field_room_number:0) for the delta on the field.