Rpod 172 top bunk

Last month I took the top bunk out (well cut 1/2 of it out) and left the part next to the bathroom as a shelf.  I was surprised by the lack of inner supporting structure for the whole top bunk and added a center support for the remaining "shelf" I left in place.  If you pull the top wooden panel off, mine wasn't glued just tacked in place, you'll see the skeleton.  I think the idea by Marwayne might work well for you.  You can always add more cross members if needed.  To be sure they are minimal from the factory.

I though about this for my 182G, but never implemented it. A ladder that is not only for climbing onto the upper bunk, but stationary once you get up into the bunk and has some structural support to the floor would help in the unsupported areas.  Enforcing the existing supports would also be in order. I'm not sure how you would qualify the supported weight, but if it were me, I'd make sure no one is below me when I put it to use for the first time.