Hmm, some interesting answers. I would never have thought anyone would leave a pet loose in the trailer while underway. Thinking about it now I guess I can possibly see cats doing quite well but I just don't buy it for dogs. Such different animals. Having spent my career working with animals I would disagree with the practice for dogs.
My wife tried it once for 2 miles and reported a very bumpy and disorienting ride.
Our little girl stays quite comfortable in the back seat of the crew cab, harnessed in to her car seat. It's also convenient if we have to get out at a stop, we just leave the old diesel running with the A/C on for her.