Thanks for the replies and links guys.
I'm just about to buy an Express passenger van with driver side passenger doors, so the engineered framing is already there. I was thinking I could weld the door together and mount them on a slide-out. I'd have to figure some kind of latch mechanism but that's a detail for later.
The opening is +-50". Wide enough for a bed? Definitely wide enough for a sofa or a kitchenette. It could be as deep as the width of the driver's seat.
The wheels are turning, but I need to have truck in my driveway before I actually decide on anything. We will decide on a floor plan when we get it. We'll start off with a temporary bed in the rear and see how that works out over the summer.