On our last camping trip, we decided the allow the dogs to ride in the travel trailer on the way to our campsite, so we could fit my daughters friend in the truck to come on the trip. We opened a window towards the front and used a baby gate to keep them in the back bunk area. When we arrived at the campsite about 2 hrs away (with traffic), we began hooking up when I tapped on the window and the dogs barely moved (when normally they would've been wagging their entire bodies.)
Then we heard it.. the carbon monoxide alarm. I could not get in fast enough. The dogs were all panting and lethargic. I got them out and was probably in mild shock myself. It took a few minutes to air everything out and their behavior to normalize. When everything settled down I began to tear up thinking we could have lost all 3 dogs in one pop and would have had an entirely different trip, and life changing event occur.
Either in the truck with us (especially if its a warm day) or windows closed in the travel trailer, plus stopping every 45 minutes to check on them.