I really appreciate your concern. However, in our case, I drive for up to 10, sometimes 11 hours per day. When I stop for breaks, it's only for about 10-15 minutes to stretch and pee. There is a bed behind the seats and that's where AJ spends most of his time while we're moving.
If we get into an accident, whether we are restrained or not, most likely, due to the weight and construction of the vehicle, neither of us would survive anyway. Therefore, after weighing the risk of injury vs. benefit of letting him choose where he's most comfortable, I let him choose. He knows where not to be when the truck is moving and (IMO) it would be cruel to crate or tie him for those number of hours per day.
I do wear a seatbelt at all times, but this is because of a habit I developed driving regular cars. I feel weird without it now. He gets to stand up, move around, stretch, get a drink of water….Unless there is a way to restrain him without cruelty, he will continue to have run of the areas of the truck not necessary to its operation while we are moving.
I have the passenger side built up with a document case, a duffel bag, my laptop computer and some rugs that I throw on the dash so he can lay up there while we're sitting. All that stuff stacked up makes a fairly level area he can bed down on if he wants to.