The floor is a “sandwich” with 3/4 inch plywood on the top and bottom, and 2 by 4 framing between, with hard foam insulation inside the framing. The bottom plywood was painted with water proof sealant, and bolted down to the trailer with 100 bolts. This required drilling 100 holes in the metal trailer first.
A photo of the hard insulation that is inside the floor “sandwich.” The 2 inch wide plus 1 and 1/2 inch wide insulation adds up to the 3 and 1/2 inch actual width of the 2 by 4 wood. (So it fills up the space completely.)