Pet kennel. I use one like this for long-distance travel with my cats. It fits nicely in the back seat and one side can be used for sleeping and the other side for a little high-sided litterbox and a little food. You can put a little water on that side as well, but you need a container that can tolerate sloshing!
Things to consider-sloped floor with drain(s), overhead storage, sinks, hideaway trash bins (probably to store dog food in), separate area (with couch) for sitting...think a hide a bed for whelping time would be important