We have a 76 31' Excello with a rear bedroom that had to be gutted and redone. After taking several measurements sideways, lenth-wise, diagnal, etc, we finally settled on an Ikea sofa bed. The one we got was PS Lovet (or other names depending on what density of mattress you get). It unfolds really easily into a basically queen sized bed and leaves a little bit of room to go around it if you go sideways around it. It folds easily too, and has wheels on it so you can maneuver it to use either as a bed or as a couch. We actually bought another one to replace the missing gaucho in the front of the trailer too, took the trailer out with some willing friends (test subjects), and everybody actually slept really well.
Here's the link: IKEA | Sofas & armchairs | Sofa beds | IKEA PS LÖVÅS | Sofa bed
Oh and the very best part is it comes in pieces (easy to put together) that can actually get through the trailer door.