You got a couple of "most common" choices and both are messy unfortunately. I worked with Peggy bout five years ago when I rebuilt mine. So my take and 10 years of owning a Vacuflush is. Right below the ball valve where the waste path goes, it makes a 90 degree turn, toilet paper that didnt quite desolve can get stuck. Its a small hole. Bernoulis Theory backwards~ Waste that stays in the toilet when you flush, is usually what I am about to describe. And unclogging involves undoing the line that goes into the bottom of the toilet, because it is very hard to get anything down in there to release the clog. I will cut off pump and water pressure, bail out and soak up all the water in toilet cuz anything left is gonna wind up on your floor and on your shoes so be ready! Then I carefully push what I can back into the toilet and get a pair of forceps, or needle nose, hold the ball valve open and dig it out bit by bit. The other place is at the duckbill valves. There are two upstream and downstream, right before and after the pump in the lines. If everything hits at once it can hang up in them and they will not close. Thus, three seconds for #1 and at least ten seconds for #2. The duckbill valves must close for the vacuum to build back up. They get rounded over time also. How long since you changed them out? Not a difficult job if your pump is easy to get too. Finally(just remembered this one), although not that common, if your plumbing makes a tight 180 or 90 degree turn on the way to the vacuum tank, lines can get soft and collapse or kink just alike a garden hose. The little pump builds up a lot of vacuum. Been there seen that on a slipmates head.
So those places are the most common for a clog. The duckbill valves are a culprit for a pump that wont shut off, but generally with the assumption that the head flushed first. Also on a side note, if your ball valve is not sealing, the pump will run and vacuum will not build up but not very common.
When you flush, is the water just sitting?
Good news is your pump is running and IMHO, they are just about indestructable. And some times you get lucky if you just let it sit for a while and get soft.
Hopefully no one flushed anything but TP(single ply or marine grade) and body waste. If a feminine product went down it, or a face wipe, paper towel, etc, good luck.
Hope this helps. THe most important safety brief you can give to guests is where the life jackets are and how to USE the head!
Edit. If you do choose to investigate, wear rubber gloves, have plenty of paper towels or rags ready and in place below lines going into pumps etc, cause you dont want fluid from lines getting into places where it cant get cleaned up. I personally cut a piece of a plastic bag to work as a tarp and then put rags on top. That way hopefully nothing gets on any surfaces or you get a boat that smells like a head! Plastic bags to immediately put stuff in. And dont forget to wipe down your tools when your done! Also, a first mate with camera in hand taking pics while you take it apart can be helpful. Also, if the installers didnt double clamp all the lines, now is a good time to do that.
