I made my own pull out sleeper sofa. I wanted a true 80"x60" queen bed, of which an RV type is not made. I made a base of 80"x30" with the cushion built into 3/4" plywood. The bottom pulls out with legs and brackets to the 60" position. Then the back is a folding cushion that are two 80"x15" sewn together to fold out and fill in between the wall and the now pulled out base. Works well.
You could do the same thing-except have the two 80"x15" folding back mounted on plywood with two pull out stabilizers to make a hinge up bunk bed. Good Luck, TomC