So, the problem with these foam cradles is that they do not affix the kayak to the top of the camper. They just sit under the boat to provide a bit of protection between the boat and the roof. You still need to tie down the boat to something and depending on the configuration of the roof of your camper, you may not have a lot to tie into.
My kayaks are 17 to 18 feet long. In the back of my camper roof I have the built-in camper rack system, the racks are on the sides and trying to affix the kayak using ropes would allows for too movement of the boat. In the front, there is nothing on top to tie into, so I would have to run a rope down to the front bumper, or to the underside of the camper's cab over. None of these options is secure enough in my situation. Just think of those times when your driving your TC down a superhighway and you hit one of those severe dips in the road that sends the whole camper back and forth... I want my boats to feel like they are fully connected to the roof with no wiggling that might damage the kayak, the roof, or something on the roof. To achieve this, I bit the bullet and purchased the necessary Yakima gear. I've taken several thousand-mile trips with the kayaks loaded with no problems what-so-ever.
Also, in my case, drilling the roof was no big deal. My roof is metal and fairly sturdy. I had to add two Yakima 54" tracks (
http://yakima.com/shop/racksystems/permanent-installation/tracks54-w-60hardware) to either side of my roof. I think I drilled 4 holes per side for a total of 8 holes. Prior to installing the tracks, I gobbed down some dicor roof sealer into each hole and down the entire length of the track as well as on top of the screw from the top side of the yakima track. I've had no problems with leaks. I built on top of this with a set of Yakima landing pads, control towers, cross bars, and kayak saddles.
It wasn't cheap, but it did the job.
Greg