I've traveled with one indoor/outdoor cat for several years and this year I have added two more!
I was concerned too, before I traveled with cats, and I did some reading online. The best tip I ever read was: Traveling with a cat will change what campsites you choose. Cats need a place that has some cover. They need bushes or tall grass or something that they can hide in. A park-like campground with low grass or dirt and limbed up trees is terrifying to cats. They have nothing to hide in.
Harnesses are scary for cats, for the same reason. They feel they could not easily run away and up a tree if a dog came along. My one cat tolerates the harness only because he loves me. But he reminds me that it is much less safe for him than being free. I haven't tried it on the other cats. I only use a harness if we are at a campground that requires it, and we haven't been in one this year.
Each cat is different, and you will have to learn how camping works with your cats.
I have one cat who is very brave and attacked a Great Dane puppy. He wanted to attack other dogs, but I think I have talked him out of the advisability of that.
I kept him in the first few nights at our Barnyard location, just letting him out during the day, but he really wanted to be out and eventually I let him stay out at night. This was more about my trust in other humans than in him. I worried about whether they would be careful, driving at night.
Another cat, who was a 100% outdoor cat at my S&B, once I brought her in the van this year she refused to leave for the first 7 days. Then when she finally did leave, I thought she'd just found a new hiding place in the van and didn't realize she was outside. (I feel bad about this, but she had been so adamant about staying in that I didn't think otherwise.)
She spent the night outside, and didn't come find me until 3 in the afternoon the next day. I'm not sure how her night went, but she was really happy when I saw her in the afternoon and she has now decided that she has a 100% outdoor cat again. She doesn't even want to come in the van to eat, so I have started feeding her outside.
My old man cat, in the one I have traveled with for several years,loves boondocking. He loves to hunt the wild rodents all night. But now, camped in a farm-type atmosphere, he has followed his S&B pattern of being out during the day and sleeping in the van with me at night.
When we boondock, he stays out till 2 in the morning, but he comes by the van every half hour to let me know he's ok and having a fantastic time. He knows I worry :-). He also hops in the van if something spooks him.
I love my cats enormously, and some people might think that I am endangering them by letting them go out. But it's so wonderful to see them happy, and they love to be outside.