Only harness my girls have never been able to get out of is called a comfort harness. it clips right between the shoulders. I got them in the dog section of Petsmart where the basic harnesses, collars, and leashes are, nothing fancy.
May I suggest if you are taking your kitty camping, you take a hard kennel or puppy playyard. I think it would be less stressful on your kitty to have a safe area than being tied. You could always make one with a little bit of fencing and clip rings.