Putting a solid band around a neck and tying it off to a moving vehicle WILL result in major neck injury if there is a sudden stop. The dog loose in a camper shell MAY just result in the dog falling down off its feet with little impact if there is a sudden stop.
I have seen too many dogs tied to the bed of a truck and having their neck be the best focus of every stop or slow down. Imagine how uncomfortable that must feel - at the very least.
Personally with extra, king, crew, etc., cabs these days, there is really no reason for a dog to be in the bed of a truck. (Not discussing working dogs in the performance of their duties.)