So, I went ahead and ordered the combo adaptor. I should be set for any eventuality.
...Except there are also not only the 6-pin rounds, but evidently 5-pin round plugs and sockets are used in this app as well.
Ah, well. My intent to "be prepared" only goes so far...
If you ever get a trailer with a strange plug, you can always just put on a plug that matches your vehicle connector.