I spent A LOT of time thinking about the Murphy bed setup. I wanted something that was sturdy and comfortable, but does not have a large footprint or is overly complicated.
I found google images to be extremely helpful in searching various murphy bed configurations. There are all sorts of designs with struts, springs, various kinds of hinges and a multitude of outer supports (shelf, fold out foot, etc., etc.)
I started by recognizing that a sturdy, but lightweight bedframe would be a good first step.
I used 14Ga 1"x1" steel to make a bedframe. Measuring 14 times and cutting once....okay, sometimes I junked a few pieces, I started like this.

It is a queen frame that is 82" long and 62" wide. A queen mattress is ~80" by 60", so I wanted to have a few extra inches for the sheets and bedding.
I used a Mig welder. My other welding projects prior to this include a welding cart (looks terrible) and a bed-side table for my wife that is basically a cube. Patience is key.
Eventually my welds got better.

I welded all around each connection and only ground down the ones where I would later mount 1/2" plywood as a base for the mattress.

After staring at it for several days I came to the realization that a little more reinforcement might be necessary. I don't want to hear a crack in the middle of the night and find myself in a heap of wood and metal.
Here is the in bed-frame before I rattle canned it with black spray-paint.

Other items to sort out along the way on the Murphy bed were...
What type of mattress?
Hinge / mount design?
How to fold it up into the side of the trailer?
Will this thing support 400 lbs? (2 adults)
Does it fit parallel or perpendicular to the trailer?
To be continued...