We went back and forth trying to plan a simple, rigid bed that maximized useful space below. We came to a simple and elegant solution.
We mounted a piece of angle iron to each wall of the bus with a screw in each of the steel studs.
This provided a stable shelf to support our bed frame.
Once we had a stable base, we framed the bed platform with 2x4s and quality plywood. This is subtly different from framing a wall of a house. In a house, you have a sill and a plate that run the full width of the wall. On the bus, the “studs” are full length so that the end of each is supported directly on the angle iron. We made two pieces rather than one as the platform is king-sized and therefore larger than a sheet of plywood. This makes the whole platform removable if we need access to this giant area.
Two screws into each platform through the bottom of of the angle iron are sufficient to secure the bed.
Notice that the top of the platform is aligned to the bottom of the window. This is maximizes the verticle space above the bed and gives a huge clear space below as a play area or extra storage. We could even roll out a few extra sleeping bags if we needed the bus to sleep even more people!