If I can find a ready made couch/bed that will work, I'll probably use it too just to cut down on build time, especially since I'm not making much progress as it is. One thing that is limiting me, is the space to put it is only 6' long exactly. I have a tall external compartment in the right rear corner, and I do not want to sacrifice it for the inside space. It was the backboard storage compartment and is just over 6" tall, so it's perfect for storing anything long like awning poles, flag poles, beach umbrellas, hi-lift jack, shovels, etc.
To me, the outside storage compartments are one of the biggest advantages of using an ambulance as a camper. Most campers this size have very little or no outside storage at all. A lot of people install water tanks, propane cylinders, generators, etc. in their compartments and loose that valuable space, but there simply isn't anywhere else to mount these items. I will be installing a Honda EU3000i generator in one of mine.
In my case, since I'm building a camper trailer out of the rear half of a Type III ambulance, I have removed everything underneath that's not needed, therefore the entire space between the frame rails is free and that's where I plan on mounting my water tanks, etc.