maybe add some heavy duty pressure treated lumber?

Lance actually has holes in bottom floor.
Maybe not design for drainage, but water tank drain fittings go into the floor holes and all is covered with flashing hold by couple of screws on the bottom side.
Can't say I am in favor of this design, but it works for most.
x2 on composite decking not only being heavy, but extremely slippery.
Not much structural strength in it neither.