Here's how I did mine on my last camper. I no longer have this camper, but I did keep the tube, it's tucked way in my utility shed at home.
The pipe is PVC pipe, the slide tray is a PVC pipe, much thinner, I split long ways and rounded the corners. The tray (2 halves), also served as a support for the hoses when the sewer hose was attached to the trailer. The hoses would not drag the ground this way. (funny, I've been using the same hoses for about 7 years now, and no holes yet.
This is a 10 foot tube (too long to go cross wise under the camper), so I installed in long-wise. The end of the tub touched the frame support nearest the rear axle. Both end had bath tub drains in them so air would pass through and dry everything out when traveling. I had to put a larger end on it, so I would not have to disconnect the connectors all the time.
You said you wanted the square fence post style, but thought I share my design anyway. You may be able to incorporate something from it.
My current trailer has an enclosed underbelly. So I'm not sure how to attach this pipe now. After 2 years, I still have not put it on my current camper. I use a plastic steamer trunk to haul my sewer hoses now, and all my metal rods and poles slide in the trailer bumper. So far, I've not had a need for this, so it's still hanging in my shed:
Considering I have 3 holding tanks on my current tamper, and 2 outlets, I have enough hose to connect them together with a Y and bring them to one outlet. Too much hose now for the old PVC pipe.