3/4" ply with "joists" should do it easily - just like the floor in your house. Depending on your weight, I think you could get away with 1x3 frame, 3/4 ply on top, and a "skirt" board around the perimeter & a piano hinge screwd into the bottom of your tube rails & the bottom of the skirt board. You could make the frame the same size as your steel perimeter & put weatherstripping on top of the 1" square tube to seal it up. All very clear in my head. I could do drawings if you're confused.
Better yet, it might be worthwhile to have a welder make a steel frame from 1" square tube, that matches the dimension of your trailer. Once again, weatherstrip between the two mating surfaces, and 3/4" ply on top. You should be able to hire the steel framework out for under $150, easily. Weld-on hinges are available, and I'd suspect 3 on one side of a steel frame should do the trick with no trouble.