I used a round papa-san mattress when I lived in my van. In the day, it was a couch. I used a foam inset to make it long enough to actually be a mattress, but it was quite comfortable.
Trobo! I was never able to get Cocobutt on the Kuranda but it supported me just fine (180 lbs). It was the aluminum and heavy duty vinyl model with a slip cover, ~$150.
Her current bed is $60 round futon cushion:
http://www.amazon.com/Cotton-Craft-Chocolate-Overstuffed-Comfortable/dp/B00GMHWXBY/
with a washable cover:
http://www.amazon.com/Brown-Papasan-Cushion-Cover-ties/dp/B00HGT1GXI/ (this shit is overpriced in my opinion but it's apparently not a very common product so I had trouble finding it for cheaper elsewhere.)
She'll actually use it occasionally (pretty much only when there's no room for her on the couch)