Every video I saw online showed people building a bed supported by vertical wooden posts drilled into the wall. But I wanted to build a shelf bed like I had seen in photos. The frame of my van had holes I was able to use, so i used a 2x6 for a ledger board, and bolted it in to the holes in the metal frame. Then I lay three 2x4’s on top like joists (notched them in to the legerboards). And then I put some thin bed slats over those three joists and put a mattress on it. It works great and I love the empty space below. I used bolts and nuts, because I was able to reach the inside of the metal frame, but you could probably use self-tapping metal screws if they were really strong? Or drill your own holes and then use bolts and nuts.