[JRTJH]

It would not extend through the extendable bumper, if it did, it would be damaged when you pull the bumper out. So, it's either above the bumper or possibly it's on the side of the frame rail/trailer floor, outside the frame rail. Ours is centered on the bottom of the fresh water tank, about 1' inboard from the trailer frame I beam. I've seen several trailers where the drain is on the outside of the I beam, located between the sidewall skirt and the I beam.

There should be some spray expanding foam to seal the hole in the coroplast where PEX tubing comes through, so maybe locating a "blob of expanding foam" would give you an indication where it is.