It really depends on your animals.
In the tow vehicle is safer.
Of course it can also be noisier.
We used to travel with two Siamese, when they were young they didn't "hate" traveling inside the trailer. When they got old, they hated it. But then they also just endured traveling in the truck.
Of course "in the tow vehicle" means more stops so that can wander around the rig deciding to use the liter box. So then you give up and hit the road again, to discover (10 minutes later) now HAS TO GO.
We are still working on options with the new cats