I have details on a fatal crash involving two professional cowboys about 10 years ago... Long (sad) story short; the driver of a crew cab dually pickup (Shaun McMullen) died when a woman crossed the center line and hit them head on at full speed.

He was wearing a seat belt and had an air bag. Stran Smith, his hauling partner was asleep in the nose of a living quarters horsetrailer, and wasn't hurt.
I know that horse trailers are generally heavier built than travel trailers, so that may have played a role. However, one thing to consider: The more "crush zone" you have, the more shock absorption there is. This may have been what saved Stran, as he was about 15 feet from the impact. In fact, in all the crashes I know of where someone was in the LQ of a trailer, the occupants of the trailer came out better than those in the pickup.
On the other hand, in a roll-over, you are probably better off in the tow vehicle.
There are other hazards while riding in a trailer however, such as asphyxiation, fire, or medical emergency. This is why it's important to have communication with the driver.
The rules do vary from state-to-state, and as someone mentioned above, the officer may or may-not know the rules very well. Inform yourself on the regs, and make your own decision about safety.