While I understand how plugged vent might prevent emptying a tank I wonder how the toilet would work in that situation too.
Flushing a toilet against a blocked vent will pressurize the system. Opening the deck pumpout cap when the tank is pressurized will release a geyser. The first clue that the system is becoming pressurized is a toilet that starts "burping" and spitting up when you flush it.
Then there is the question of wouldn't the tank simply pull air through the toilet itself if the vent were plugged and the tank being pumped?
It'll try to pull in air from anywhere it can, but the air flow through the toilet isn't enough to prevent a strong pumpout from pulling some vacuum...iow, it may be able to pull the waste out, but very slowly with pressure that can suck tank walls in. A macerator pump doesn't have enough suction to pull much of anything out against a blocked vent. In fact, any hissing in a toilet bowl is a strong indication of a blocked vent.
Our Jabsco not-so-Quiet-flush only has a single joker valve and a small standing column of water to pull through...don't these tank pumps have enough suction to pull air through that?
If the joker is so worn that the slit has become a hole, the pumpout can empty the tank, but very slowly while it pulls on the tank walls. In fact, if the boat/toilets sit unused long enough, the pressure in the system can escape through the toilet...allowing the deck pumpout cap to be opened without releasing a geyser. But it's unlikely the toilet(s) on a liveaboard will sit unused long enough to do that.
As with most things, prevention is a lot easier--and in the case of a blocked tank vent, more pleasant--than cure. And the simplest way to prevent a blocked holding tank vent is to replace the "vent" thru-hull (which is actually designed for fuel tank vents, but boat builders use the same thru-hulls on all vent lines) with an open "bulkhead" or "mushroom" thru-hull that you can stick a hose nozzle up against and backflush the vent line every time you wash the boat and/or pump out.
I believe that the tank might be sludge filled...
If you've never flushed out the tank, there could be an inch or two of sludge on the bottom, but no more...and not even that much unless you use a lethal chemical tank product that kills the bacteria needed to break down and liquify solids and TP. Unlike RVs, the contents of marine tanks is mostly flush water...RV toilets use only enough to rinse the bowl as the the contents slide into the tank directly below it--just a pint or two...and most RVers do use lethal chemicals, primarily Thetford products. So 90% of the contents of an RV tank IS sludge. Plus, RV tanks are literally dumped out a port in the bottom of the tank...no rinsing--RVers are even lazier about tank maintenance than boat owners...leaving what's known as the "poop pyramid" to build up in the tanks. When the RV sits between trips, the sludge turns to concrete. RVers don't do anything to PREVENT it, but they do get creative in their efforts to cure it with the least amount of effort possible.
Use a bio-active or other tank product that doesn't kill all the bacteria needed to break down and liquify solids and TP...rinse out your tank 2-3 x a year--and especially before winter or any other extended layup--and sludge won't be a problem.
And btw, joker valves should be replaced at least every two years if you want them to be a one-way valve instead of just a piece of rubber that partially blocks the flow from the toilet.
Peggie
http://www.amazon.com/New-Get-Rid-Boat-Odors/dp/1892399784/
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein