My trailer has channel iron for crossmembers, so drilling up from the bottom is only an option if you don't care if the screw heads are crooked where they come out of the deck. To get the screw head flat it requires installation from the top. You could lay the boards down, mark them from the underside, then flip them to drill, flip them back over and install the screw, but I think it is easier to use 2 drill motors, one with a drill bit and the other with a #50 Torx bit in it. Lay the board down, drill it, drop a self tapping screw into the hole and hit it with the other drill motor, then go onto the next one.
David from jax