On our 2014 Voltage...the OSB platform the mattress sits on...the Passenger side of the OSB came within 2" of the edge of the mattress. On the Driver side, my side, the mattress hung over the OSB by 8". No wonder I felt I was always under the edge of the night stand!!
You could, as I began thinking yesterday...with some sag in the middle. Actually frame a "wall" screwed to the floor that the OSB comes to rest on when the bed is down. Might lose a little storage. But I'm thinking of sheeting it with 1/8" like the rest of the trailer, and that would help keep little stuff from sliding around under the bed.
I also used a bit of 1"x1" and framed three sides of a 4" square, 18" towards the head of the bed, from the foot, so that I can use a 42" piece of 1"x2" hardwood to prop the bed up when loading or unloading. Ours, if the bed is made, the gas cylinders won't hold it up anyway. The ground end of the 1"x2" rests in the corner of the bed base on the Driver side.