In our older Jayco (2003) the cushion has plywood inside it to span the cabover cutout, so it would not be possible to slide something between the cushion and the supporting material.
I'd start out with an ordinary stepladder. Maybe a wooden one since it'd be easier to modify than an aluminum one.
Is that area of the floor carpeted? If so a ladder set on the carpet and leaning against the bunk won't slip.
I don't like most of the "RV Bunk Ladders" because the rungs are very narrow and that makes them painful on bare feet. Stepladder has wide surfaces.