I'm posting this really just because when I was searching, I didn't find all the info I was looking for as easily as I thought I would.
I recently bought a Ruggedized Kukenam Sky RTT from Tepui. I plan to eventually get a full length rack, so I didn't want to waste the money getting some Yakima or Thule (or anything else) mounts and crossbars. After doing some research, it seems like the general consensus is that the stock crossbars aren't up to the task of supporting 180 pounds (that's how much this one weighs) but that the stock rails (the part that runs parallel to the car) are.
All that being said, I set out to mount this tent to my car by using the rails.
The only thing that's done differently is that the mounting channels that come with the tent are turned 90 degrees from their instructed way and they stick out just a little bit on either side from the base of the tent (maybe 5-6 inches on both sides).
I was able to mount the tent without having to take off the stock crossbars but the stock crossbars may end up having a slight load on them when the tent is occupied due to the stock crossbars having a slight bow to them.
I was in a hurry today and had other errands to take care of so I didn't have time to tent it all up properly but here's a quick shot.
IMG_20161204_122714 by
adllewis2011, on Flickr