Insulation in the floor has been an ongoing concern since acquiring the trailer. I plan to use 1.5" white foam board between the existing 2x6's planks that came with the trailer. Four planks are placed 24" on center, leaving about 16.5" between for 1 1/2" foam insulation sheets. This will supplement the 1.5" additional foam that will be placed on top of the planks after the 2x6 sills are screwed into the frame with self-tapping screws. I'm thinking about bolting these into the frame as well. In essence there will be roughly 3" of insulation in the floor which I expect is sufficient for a TH located in the sun belt. However, I still have plans to add additional foam insulation (sheets or sprayed) within the C-channels and then screw aluminum sheathing to the bottom of the C-channel to protection from moisture from the road and rodents.
Also, we verified that the trailer frame is a perfect rectangle...the diagonal measurements were identical! Yea!