Great news! Thanks for the update.
It sounds like the plunger worked. That is why mine is always on board.
Everything I do is frequently "over the top", including preventative maintenance on the head.
Depending on usage, either annually or every other year, I disassemble, inspect, clean and reassemble the head components, to ensure ongoing proper operation.
That starts w/ a complete pump out, then I remove the tank level sensor, by unscrewing from the top. This creates a 4" opening. I modified the wiring harness for this level sensor to add a bulk inline connector, so I can unplug and remove the sensor assembly from the boat.
Using a looong / large bottle brush, powdered cleaner and a garden hose I scrub the internal walls of the holding tank, while the macerator is set to continuously discharge.
The level sensors are cleaned at that time.
My VG2 vacuum generator is removed, disassembled, cleaned, inspected and reassembled. So far I have not disassembled the diaphragm.
The duckbill Joker valves are replaced when they start becoming hard (not as supple as new).
If effecting this service, do not over tighten the fittings retaining the duckbill valves. That will cause them to have a continuous slight opening, leaking vacuum, causing excessive cycling. The fittings only need to be lightly snug.
Not sure if this will restore the missing sucking sound, but this has helped my system remain in good shape.