What about what this guy did - remember this video posted recently on another thread?
Man Turns Garbage Truck into Unbelievable Boat RV | Mega RV Countdown | ulive
Go to 1:54 in the video.
Instead of going double-high for the whole thing, he built "down" just for the bunks. I've been thinking about doing the same thing - on one side, where you'd normally have belly storage, setting up a 6'-8' section that goes "down" into what would normally be that storage bay. I'd frame it with angle-iron and insulate its floor and walls the same way as the rest.
You wouldn't get nearly as much space out of this as a full lift/drop, but I think it would be a lot easier to engineer a single reasonably-sized bay. For me, that would be the kids' bunks, to fit one more bunk into that area.
I wouldn't want anybody in there while traveling in case you get T-boned but it seems like an easy win if you just want more interior storage or an extra bunk.