With four people, I'd go for a rig no smaller than 30 feet, and some cars can be adapted with a lubricant pump for towing on all four wheels.
I have seen some very well laid out "C" floor plans with multiple sleeping surfaces so everyone in a four member family gets a bed if they so chose. One plan even had a seat and a place to play console video games inside, when weather was not friendly to being outside.
I would highly suggest slides. Yes, they have drawbacks, but the space they give can mean a very pleasant trip without the feeling of being cramped.