We built a dinette that folds down into a bed and added a bunk option above it. You can seen the blog post if you click here
I realize as I look at it again that I have no pictures of the bunk support boards on the walls at each end of the dinette. They are 1x2 birch to match the plywood walls, bolted through those walls in three places to help distribute the weight. The bunk bed platform fits snugly against those walls and bolts down through the support boards in two places. This makes the walls and bunk platform into a single, solid support system.
We replaced our interior bulkhead dark brown plastic covered walls with real birch plywood which has a bit more strength. Our bunkbed easily supports our adult daughter.