The subject of kayak racks comes up regularly around here, so last week when I saw someone in our campground with a nice solution, I shot some photos and spoke with the owner. Unfortunately these were home fabricated, not a purchased solution, so you would have some homework to do. But I have seen similar things on camper shells, so you might be able to purchase some portion off the shelf.
These are 12' kayaks, and they are the Hobie fishing model with pedal paddles. Owner says this is his second fiver with this type of rack, and that he has not had any issues. This was about a 30' fiver.
The brackets are bolted into the top frame rail. You can see the row of 3 lag bolts just below the drip rail. The lower screws are not into wall structure, so they don't support a lot, but they do add a little torsional resistance.
There is a spacer about 1/2" thick, plus a little joggle bends in the plate, so that it clears the drip rail.
At the very rear, he has a roller spanning across, so that he can slide the kayaks up from the rear without dragging them on the fiver body. Takes 2 people ideally to get them up there, but his wife was all the help he needed, so it didn't seem like a heavy lift.
The gentleman happened to be a Hobie rep and was on the Hobie fishing team.
2006 F250 Diesel
2011 Keystone Cougar 278RKSWE Fiver