We have two labs and they just lay on the sofa, the bed in the back, under the dinette, or on the floor right behind the cab when we're driving. To contain them if we go out sightseeing, etc. they have crates that fit just perfectly on the dinette with the table down in "bed mode" but we don't put the cushions under their crates, just a rug to keep them from slipping when they get in and out.
When we had just one and a truck, he used to wear a harness seatbelt. So I know we're not as safety conscious as we used to be with him.