The bumper is a bad place to store the sewer hose. The interior walls will rust and the vibrations from going down the road will make pinholes in the hose.
Some people use a 6 inch square PVC fence post and caps from HD or Lowes and mount it on the rear bumper with straps I use a round one like in the link and just drove self tapping screws into the bumper. I am able to fit 20 ft of hose without adapters. I have a small tub with lid that stores the adapters and a rubber donut (which I have never used)
Link Also get a 90 degree hose fitting to attach between the fresh water hose and the trailer.
Link