Good question, and I don't have a definitive answer, but my past experience with the hollow square bumper was not good.
Several years ago, I had both a spare tire and bicycle rack mounted on my bumper. The rack held four bicycles. The subsequent bouncing of the bikes over only half a season of camping cracked the welds on both ends of the bumper.
As a result, I had a hitch receiver custom made and the bumper re-welded to accommodate the bicycle rack.
In short, I would be by careful with mounting any significant weight if it is a hollow bumper that accommodates your sewer hose.
Happy camping !