My camper van has one that runs lengthwise down the side behind the driver's seat. First night I tried it, was parked at the curb in front of the house...
The street is crowned. The van was tilted a bit toward the curb. I spent the whole bloody night scootching back up from the edge so I didn't roll off into the aisle.
When the van is level though, it's fine. Mine is 6'4" long, and so am I and I don't feel like it's too short (back of driver's seat at one end, floor to ceiling cabinet at the other). Way back in high school, I had a "hippie van" that had a 6 footer across the back. To sleep on it, I had to be catty corner, but it was fine as well.
One thing is that the one I have now, the bottom part is on springs, but the back part isn't - it's just a thick pad of foam. So there is a difference in the give, and I tend to stay on the wider sprung part. And, since that's wide enough, I usually don't even bother folding it out and making it up anymore. I just stretch out on it as a couch and use a sleeping bag. Quick and easy.
I'm actually getting ready to rip it out and build a wooden box instead. The problem is that the bottom doesn't raise up. It does, but only a couple of inches...just enough to do the fold out thing. But under the couch is the water heater, the fresh water tank, the water pump, some plumbing - including the drain into the top of the gray water tank - as well as quite a bit of usable storage space, and I can't get to ANY of it except by laying in the aisle and reaching in from the front. And since I don't fold it out anymore anyway... I've already drawn up the plans, plotted the cuts for the plywood and sourced the foam. Might get around to it next week.
That wasn't a problem before - with the hippie van I could just open the back doors and access the area under the couch. Can't do that with this one.