Hi, Ben, welcome to the boards.

This was a recent discussion:
http://petoftheday.com/talk/Forum12/HTML/000108.html

and
http://petoftheday.com/talk/Forum9/HTML/000088.html

I would definitely not take a cat on a camping trip, even if leash trained. There would be way too many risks, on the trip up, in the camp, and on the trip back. A cat on a leash has to be supervised because of both an escape risk and a tangle and choke risk, so any time you could not have your full attention to your cat he would have to be crated. That would certainly be no fun. If your cat is like most, neither would the car ride up or back. I know you think he would enjoy it, but I can't imagine he would get much fun out of it. One last thing:
http://petoftheday.com/talk/Forum12/HTML/000023.html

The harness is a good idea if you can get it to work, but I hope you think long and hard on the camping cat idea and make sure it will work before you do it.