SmokeyBob I'd check out your bed size before I get much farther. I built a single bed/couch in my trailer that is 5'wide. The bed is 30" wide and I'd really like it 2-3" wider. Minimum size I'd say you could use at all is about 27" wide (if you're small) and with two of those in a 5' wide trailer that leaves a 6" aisle not accounting for any wall thickness.
I think I would build 6' wide over the axles to gain the extra space. Even in mine I now wish I'd gone a bit wider and completly covered the fender for the extra room inside.
Here is finished interior:
And here is during construction, you can see how little floor space is left with a 30" bed in a 5' trailer
