Thr MFG couldn't refer you to a local authorized repair facility?
You should call the folks at Svendson's. Even if they don't work on those units, they might know who does.
Your best bet is to have the unit physically checked out by someone that knows the system. You are like a dog chasing its tail w/o having someone that knows about it taking a look at it in order to solve your issue. You can play the guessing game (at you expense) all day long and in the end, still not have it fixed.
As a side note, if it can't be fixed, I would recommend replacing it with a Sealand Vacuflush system, but hey, that is just me. I've had five new boats with that system dating back to 1987 and I never had an issue with any of them.
Good luck!