Let me try to explain more clearly...

1.  There are two pieces to the front bench seat: a bottom cushion, and a back cushion. (Actually, there is a third: a smaller filler cushion behind, along the front of the trailer.)

2.  The back cushion has an insert of 3/4" plywood.  Usually it is hinged to a small shelf and swings up to make bunk beds.  We no longer need bunk beds, so we have unhinged it. (To do this, lift it into the bunk position...to access the screws holding the two hinges in place; they are screwed to the bottom of the shelf.) 

3.  We have removed the hinges from the back of the cushion...as they are no longer needed, and so they won't cause damage. 

4.  To make the front bench seat, we simply stand the back cushion in its usual position.  It "stays put" without the hinges, without any problem. 

5.  To make up the "wider bed," first remove the bottom cushion and set it aside temporarily.  (We stand it against the table.) 

6. Next, slide the back cushion down and forward onto the fiberglass cabinet... into the place where the bottom cushion was located. (Leave in place the small filler cushion against the wall.)  In the third photo, the zippered cushion on the bottom of the bed is the (former) seat back.

7. Now, place the bottom cushion on top of the back cushion.  By way of analogy with a standard bed, in this arrangement  the back cushion (with the plywood insert) serves as the "box" and the bottom cushion as the "mattress."

8. Already, you have a wider bed...extending all the way to the front wall of the trailer, not merely to the back cushion.  But, with its 3/4" insert, the bottom cushion can be cantilevered over the edge of the fiberglass cabinet...to the desired width (as much as 36", which is standard twin bed width); we have settled on 33" (for space and comfort reasons).

9.  Cut filler foam to fit the space against the front of the trailer, to finish out your new wider bed.  Note also our suggestions above re use of a blanket and foam-filled mattress cover to smooth out the gaps between pieces and make up a comfortable bed.

10. Note: We have friends who have removed the back cushion altogether.  They had a custom bottom cushion made up that extends all the way to the front of the trailer...making for a "permanent" front bed of wider proportions.  This eliminates a good front bench seat, but large pillows could be used for back support.

I trust this helps!