The most common location for a tank vent blockage, especially in the spring when the mud daubers are building nests in any cavity they can find, is the vent thru-hull. So before you backflush the vent, use a screwdriver blade or whatever you have handy that'll work
to scrape out the thru-hull. If there's a screen
in it, knock it out...screens cause more problems than they prevent.
The second most common location on a sailboat is the connection to the tank--both that end of the vent line and the tank vent fitting--'cuz waste spills into the vent when the boat
heels and builds up to a clog. From what you're saying about access, cleaning
that out could be a problem.
A vent line filter is another possibility. If it ever gets wet, it'll block your vent line...which a big reason--along with their exhorbitant cost and short (only about a year) lifespan even if they don't get wet--why vent line filters are an even worse choice for a sailboat than they are for a powerboat. If you have one, it'll have to come out to flush out the line...up to you whether to replace it. I wouldn't.
It might be necessary--and a good idea even if it's not--to replace the vent thru-hull with an open bulkhead or "mushroom" thru-hull fitting. That makes backflushing it SOOO much easier!!
Once you get it clear, you want to keep it clear...backflushing it every time you wash the boat and/or pump
out is the way to do that.
Meanwhile, do not use the toilet again or attemp to pump
out till you've cleared the vent.