Something else, I can’t quite mark this as [solved] as I (potentially) found a kink here.
@davidsneighbour 's solution works although it only seem to display the default RSS, it completely ignores _default/single.rss.xml (and /trips/single.rss.xml and so on).
I have tried with several combinations and I can’t seem to be able to make hugo read my own RSS template, it only displays the default one.
Did I just find a bug or does anybody know if I am missing anything here?