I found one of those old-school adjustable steel bedframes in a "free" pile a few months ago. I took my angle grinder and chopped it into a custom bed size to fit the van (mattress dimensions will be 6'4" x 4'0"). I chose a 4' width as it's slightly larger than a twin and could fit 2 people, but wouldn't take up too much space otherwise. Orienting horizontally the length is a bit tight, but I'm already used to beds not being long enough for me, so I'll make do.
The amount of grinding and drilling I had to do to make this work made me question whether it was worth it (vs. just using lumber), but it's lighter, takes up less space, and it's done, so I'm happy. ha