You definitely need free airflow behind the refrigerator, vented bottom and top. There have been many posts about small fans behind the coils to increase airflow on the hot summer days as the absorption refrigerators aren't very efficient then. As long as you have it all open I would think about installing one now while it is easy. This
is but one of many threads on this site that deal with refrigerator problems and airflow.
You also need to seal this completely from the interior of the trailer to prevent CO infiltration from the flame. Basically consider this area of the trailer as outside space with as much free outside airflow as you can get.