I just purchased a new coachman 385DS diesel with bunks. We wanted a triple bunk, but our only option was normal bunk. We removed the existing bunks and install 3 bunks. One of the bunks crosses one of the windows, it opens at the bottom bunk and extends air to both beds. However the kids never open the bunk windows anyway, there is plenty of ventilation at the other side of the bunk.
If it is for the kids only. It is pretty easy. Just 3/4 inch nice plywood, brackets at both sides and the back. Make sure the brackets fit into the frame. We found ours using a magnet. A nice strip of wood to frame off the front side of the plywood and stop the mattress sliding out.
If you need more weight, you should build a frame, and put 1/4 inch plywood on the top and the bottom. We used existing frame and built a new frame for the bottom bunk.
You can buy custom size mattresses from mattress insider.
