[Tafakot]

So far, your ideas are making sense, however, with a toyhauler...theres another half bath in the "garage" area and a separate "dual holding tank (black/gray).
The "pipage" is long from both ends and gather mid rig to the final exit point.
Figuring out which is which, isn't hard to do, its more about access.
Interestingly, the valves are not even close to being in line with cables which cause uneven pulling.

Thanks to everyone for your ideas