Oh I would not use the bag at a campsite! Just for the walk from the van to the vet's door or motel room door. And maybe inside the van during travel if the harness wasn't working out. I agree that it would keep kitty from escaping real danger as well as imagined. That's also an issue with the leash. I stay by his side.
I wouldn't have him in the carrier at a campsite, either, although I might consider a wire or popup larger enclosure. Mainly, I think the leash, with an attachment point inside the van and the door open so he can dash in there if needed is going to work out.
He was attacked by a pit bull this summer and nearly died, so he and I are both very concerned and cautious about dogs.
My main question is what cheap motels might require.