I believe I read on here before that someone used [b]4 inch vinyl fence posts for a sewer hose holder that connects to the rear bumper.
That would be me. It replaced the wood 2x4 that the previous owner installed there.
The 4" size is just
big enough for the hose & fittings... I do have to feed it in carefully, though.
The curb-side end of mine looks identical to yours.
On the driver's side end, I use a rubber bumper plug from Camping World