Re: Unused Head
OK, it isn't a PortaPotti, it is a PotPourri. They typical portapotty is a self-contained unit, but the one you have uses a separate waste holding tank and apparently filters the "water" before sneding it back into the potty to be reused for the next flush.
If that's the case, the plumbing may just connect the tank and potty, with another line from the tank to a deck pump-out. That's not terribly elegant (or maybe it is) but it would keep you legal in no-discharge zones.
Rubber parts typically are rated for a five-year liftetime, even the best of them by Goodyear, Bridge/Firestone, Gates. It if was really never used, any valves or flaps in the tank should still be good but IF you can't get spares from the manufacturer? You might want to rip the whole thing out while it is still clean, and start over. If you can get spares, you still might want to break it down and replace the rubber parts, again just because it is still clean and you'll be buying time on the next replacement cycle.
Clean isn't always a big deal, with rubber gloves and chlorox and plastic bags, old used heads get worked on all the time. Still...if you can work on a clean dry system instead of playing "decontamination"...there's something to be said for that too.
Phone calls are cheap these days. I'd try calling the mfr to see if spares and a manual are still available, and if the spares aren't too pricey, think about using the first set while it is still a clean easy job.