Do you mean under 21' total length or 21' in camper length? My trailer is 22' from bumper to hitch, but only 19' of actual camper..
It has a queen bed up front, middle kitchen/dinette, rear bath.
No way to have quad bunks in this thing, unless you took out the rear bath and put the kitchen/dinette up front leaving you with a fold down bunk on the dinette.. Not sure where the bath would go??
Can't say I've seen one of those floorplans..
Back in the day when I was a kid, my folks had a 1976 Taurus that had quad bunks in the rear, middle kitchen/bath and the front was the dinette that folded down to a bed.. But, it was about 26' at least in total length..
I remember camping in a 1963 or so 12' Shasta trailer before we got the Taurus.. Had us 2 kids and Mom and Dad in that one! :)
Good luck with the search.. What I'm seeing out there now for "bunks" all seem to be over 26' to 33' in length now a days...
Mitch