Pumped my aft 25 gal holding tank after it was completely full. I know- i waited too long. Got back to my slip and the tank level shows full (99.99% sure it's not) and bowl gurgles, burps and refills part way after flushing (Jabsco manual head). I believe the issue is the vent.
Yep...you have a blocked vent, and because you do, your tank IS still full. ALWAYS watch the sight glass in the dock pumpout hose to make sure it continues to pull waste out till the tank is really empty, cease pumpout immediately if it only pulls out a little. You're lucky it wasn't strong enough to implode your tank.
The two most common locations for a vent blockage are the thru-hull and the other end of the vent line...both that end of the hose and the vent fitting on the tank.You need to clean out the thru-hull first...find a place to park the boat that will let you do that standing on the dock. You said there's a screen in the vent...knock it out--screens are spark arresters, needed on fuel vents but cause more problems than solve on waste tank vents--then use a screwdriver blade to scrape out the thru-hull.
If the only blockage is in the thru-hull (unlikely on a sailboat), waste may spew out it once you've relieved the pressure, so you'll want to be standing to the side while you scrape it out. However, unless you've used the toilet again, opening the pumpout cap to pump out the tank has already relieved the pressure.
Now you're ready to clean out the other end of the vent...remove the hose from the tank...scrape out the hose and the tank vent fitting...reconnect the vent line to the tank. Final step: stick a hose nozzle up against the vent thru-hull--which is open now that the screen is gone--and backflush the line (this is not likely accomplish anything if you do it before scraping out the thru-hull and the other end of the line). To keep the vent clear, backflush it every time you wash the boat....prevention is ALWAYS easier than cure!
As for the rest of the advice you got....
Take out that thru-fitting and put in a regular "mushroom" type thruhull fitting
Knocking the screen out of his existing vent fitting makes that unnecessary unless/until he's ready to replace it and the vent line--which would require a new larger tank fitting (which isn't hard to do)--with a larger one, 'cuz now that it's open, it can be backflushed.
Overkill and most likely unnecessary because you've already relieved the pressure when you pumped out...
With the Jabsco Head..
Flip the switch to "Dry bowl" (or to the right) and try to empty the bowl by pumping. If you can't it is plugged discharged line or more likely FULL HOLD tank.
There's a right way, a wrong way, and what some guy has gotten away with...James, you are that guy! Don't even THINK of doing that--or anything else you suggested--no matter what the toilet is, 'cuz that will only pressurize the tank, which can burst it, or--if you're lucky--only create enough back pressure for the waste to erupt in your face while you're pumping the toilet. And urine will eat through a metal internal waste tank sender, but not until after you had to remove it to clean it a couple of times.
