At this point with all the latest info. It sounds like your friends are not ready for an extensive cross country trip. For that kind of trip it will take hundreds of miles to understand and appreciate how a truck and trailer will handle in certain situations, let alone getting the hitch dialed in (that took me at least 500 miles of adjustments to get it where i was comfortable with it). And going from never towing anything to towing a combined weight of 17k plus pounds is a disaster in the making. Towing over the Excursions GVWR is not as big a deal as the complete lack of experience your friends have towing anything. I say either they start doing some local trips to get accustomed to towing or forget the trip altogether! Sorry, but that is as candy coated as i can put it. Not being mean, just honest!
Good luck!
