If you're talking about the "stink pipe" that extends up through the roof of the HiLo to vent sewer odors where you won't smell them, it does not have to be particularly airtight. In fact, since the top moves up and down, there is a section of it that "telescopes", the pipe from the bottom fits into a larger pipe at the top and the two sections slide on each other when the top is raised or lowered. There is some leakage there, but the easiest path is out the top so that's where the gasses go.
That pipe is ABS, I believe and you should be using ABS pipe in any repair to the toilet system. White pipe is PVC and it can become brittle with age. I would not use PVC piping in ANY of the plumbing systems in the trailer. Use PEX tubing for any supply lines from the fresh water tank or water heater to faucets and the toilet.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew