I used metal corrugated 29 gauge roofing material to flash the top side of my trailer where the sub-floor would sit, fastened with 1" metal to metal self taping screws. I used a metal cutting blade for my circular saw to rip a couple pieces of the roofing material lengthwise to fit on the edges.
For the subfloor, I used 2x4s at 16" spacing. The edges were blocked out to provide additional support for the walls. 1 1/2" and 2" polyisocyanurate rigid foam board was used to insulated the 3 1/2" cavity of the floor joists. I used a circular saw blade with high tooth count to cut the panels to fit between the joists. It was surprisingly easy to cut and the panels fit snug in between the joists.
Before the insulation was installed, I drilled 3/8" holes through the floor joist and through the L flange on the long sides of the trailer. A 1" hole was drilled into the floor joist a half inch to recess the bolt head to allow the sheathing to sit flush over top. More bolts were added in the middle of the floor through the floor joist and through the 2" tube steel trailer supports in the middle. A total of 26 connection points fasten the sub-floor to the trailer. I also added (4) 3/8" all thread rods through the walls from the top plate through the L flange for added support. I seen this done through Tiny Nest.
3/4" tongue and groove plywood with liquid nails adhesive, screwed into the joist at 6" spacing complete the sub-floor. Make sure to buy adhesive that won't interact with the foam insulation if you choose this sub-flooring method. I made a mistake cutting the tongue and groove and cut off the wrong side, so there's a short seam that isn't locked into place with the tongue and groove. I would suggest to consider using standard 3/4" plywood as it was suggested to me by other contractors. Or just slow down and think about your cuts using tongue and groove. The utility area of the floor joist we not sheathed or insulated to provide ventilation of the utilities and propane tanks. It took about twice as long as I thought it would which is the common theme in trying to estimating any time during the build process. Thanks to Caleb completing the base of my house.