Hi,
I have some 4" diameter black ABS pipe that has a screw on fitting on the lower end, and is open on the upper end. It is suspended from the frame with metal 'plumbers tape' that you can find at any hardware store, along with the ABS pipe and fittings. The open upper end fits snugly against one of my storage compartment doors, that is exposed to the underside of the RV in the back, as well as the water tank above the basement floor. My one fitting fits inside the 4" tube, it has a detachable 90* fitting on the end that goes into the park end of the sewer line, while the RV end will not fit into the 4" tubing, so it can not go in to far.
I wrapped the plumbers tape around the ABS twice, and it holds it snugly, without any screws into the 4" ABS. It has been mounted this way for about 6 years, and while it has drug against the ground, it has not fallen off yet.
Fred.