I have finished the floor, I used 1/2 inch OSB (oriented strand board) in two layers. The first layer was secured to the floor joists with screws every six inches and Liquid Nails. In this layer each board was placed left to right on the trailer. The second layer boards were placed perpindicular to the first layer so none of the joints matched up to prevent them from coming up.
Here is a close up image of both layers. The second layer was also secured with screws every six inches and Liquid Nails. You could use thicker OSB and make just one layer but my building inexperience led me to buy all 1/2 inch boards so some friends told me of this alternative method.
I am currently faming out the two end walls. These are probably the most difficult walls for the inexperienced because of all the angles, but once the first one is built the second goes much faster. I will post some pics and descriptions when they're finished, but until then I will leave one last picture. I saw this on a 2x4 while framing and thought it was a good sign. Have a nice day all.