On my Coachmen, the front of the bunk board doesn't really attach per se. The board itself extends to the curved portion, and then a separate, relatively light wood framework of strigners/formers shapes the curve, and then the front wall is again a rather monolithic structure (i.e. a foam sandwich with framework and such). This light framework does not attach to the bunk board—just to the outside walls and the front wall. The bunk board is presumably supported at the sides by the side framework of the cabover structure, but I haven't personally had to examine that part. (If it weren't, I am pretty sure it would end up crashing through the windshield whenever the cabover bunk is used.)
On my unit, the bottom curve framework is readily accessible from the inside by removing the bunk mattress/cushion and then removing an angled upholstered kick panel that goes between the bunk board and the front wall on the inside. This kick board is mounted with a few screws covered by upholstered snaps; about five minutes of work tops to remove it. I'm sure manufacturers and models differ in such details.
