I have been struggling to carry my AI's while towing the fifthwheel. After spending lots of money on Yakima product that never really worked I finally broke down and spent $2000 on a custom rack. Here are some photos of the solution. I can tell you that it is the most stable hauling that I have ever had. I also had the fabricator come up with 2 sliding bars that fit inside the cross bars. These slide out on either side of the truck so that I can load each yak from the side then shimmy them into position. I am currently using foam blocks on the crossbars but I am researching other options. The sails sit beside each yak and the amas go in the trailer or in the back of the truck when the trailer is disconnected. This lets me get the yaks and my trailer next to many of the great lakes for week long stays of fishing, peddling and sailing. All in all a great investment.
The biggest problem I had was positioning the yaks far enough forward to clear the front of the fifthwheel in turns. With the overhanging rack that has given me the perfect mounting points on the gunwhales to really secure the yaks without worrying about deforming the plastic in the hot Nevada sun. The yaks only extend 9" into the bed leaving plenty of clearance and only 2' in front of the truck.
The next image shows the crosstubes and the hole near the end is used to pin the sliding extension into place. It is now easy to load 1 man by lifting the front of the yak onto the extended bar then lift the back of the yak and slide the back bar out to rest it on.
The last image shows the rack detail and truck mounting. They guys that fabricated it build lots of desert racing rigs and modified trucks for the Baja and other off-road races, so they know how to mount stuff on roofs. They used stainless steel mounts located under the headliner of the truck so that their perfectly sculpted mounting legs are securely bolted on. The bolts come out when I want to take the rack off and the only thing left on the truck are the tops of the closed end, flush mounted stainless steel tubes. The whole rack weighs about 120 pounds and I not had it off since they installed it. I may never take it off as it fits in my garage and looks clean enough going down the road.