Just something to think about... In our 17' bigfoot
, I sleep on the bottom pullout sofa and my son (12 yrs, 100 lbs) has been sleeping on the bunk above it, but we are now finding that he is needing more room as far as clearance from the bunk to the roof (length is good). This is the last year for this sleeping arrangement and next year he will sleep in the front dinette bed. He hits the roof every time he turns over in his sleep (several times a night) and I wake up each time, sometimes in a panic because of the sudden noise.
In a smaller trailer, the headroom clearance will be even less than this. So I guess what I am saying is that your idea of adding a bunk could work, but not for an 'active' sleeper given the headroom restrictions.