I know this is long after the fact. However we too are a family of 5 with a 1985 Bigfoot
17' foot as well. The gaucho bunk was so close to the ceiling, that even for our smallest kid, it was uncomfortable. So like what you are suggesting, I removed the upper cabinet completely and built a permanent bunk lower down.
I built a frame near the door out of 2x4 with a ladder on the end. The bunk bottom is built the way they build Murphy beds - two sheets of 3/4" ply, with 3/4" ply spacers in between, screwed and glued (so it's 2-1/4" thick). The wall end towards the back of the trailer was tougher to build a support for, it is' a bit of an improvisation, with some supports going down to the floor.
So we sleep adults on the dinette, the middle (9) and oldest (13) on the pull out couch, and the youngest (7) on the bunk, dog (80 lbs) on the floor. Cozy is an understatement.
During the day, we cram all the bedding up in the bunk out of the way.