I would almost bet it would be the 7-way connector setup. The 7-way blade type is the standard connector the RV trailers use. I would do the 7-way setup and you can always purchase a 6-way to 7-way adapter if it turns out to be a 6-way hookup. Doing this would allow your truck setup to support the standard 7-way connections for future use.
Note the six blades and the one center pin makes up the seven connectors as shown in the above photo from google images.
Perhaps you can get your father-inlaw to take a picture of the end of the cable that plugs into the truck connector using his cell phone camera and send you the photo... The end of the connector should look just like the photo above...
I use ETRAILER.COM for all of the items I need for the trailer connectors.