We have an 85 17' Bigfoot
that had the cabinet that folds down into a bed. We have 3 children as well, so needed that fold down bunk for our oldest (now 9, but 7 when we started this).
I initially started off by cutting a sheet of plywood to fit on the fixed side of the bunk, and another piece to fit over the fold down side. I tried to strengthen the straps that hold the fold down side, but there is just so little to fasten to.
In the end though, the bunk is too close to the ceiling. My daughter couldn't sit up at all. It was designed that way so the couch below would be usable. However, with our other 2 younger kids sleeping down there, couch space is secondary. At any rate, there was no way to set it up so that my daughter could get in or out of bed herself (rather annoying for the middle of the night pee breaks) and then she fell out of bed one night while sleeping and we thought there has to be a better way.
I ended up tearing out that cabinet all together and built a self supporting bunk with a ladder up there. Renders the couch below unusable for adults, but fine for the kids. It's lower down now so has more space up top. The frame near the door is built from just 2x4s with the ladder steps built from 3/4" birch plywood. The far end (wall end) is resting upon a frame built of 1x4 to sit close to the wall. The bunk is built out of a sandwich of 3/4" birch to make it very stiff. I could have built it lighter, but I wanted it to be very sturdy.
May be more than you want to tackle, but one viewpoint from someone who's "been there, done that".