I agree, tank vent is blocked, but that's NOT
the way to check it. 'Cuz if the vent is blocked, the tank will be pressurized...Removing the vent line from the tank will send waste spewing out the vent fitting.I agree that it sounds like his tank is pressurized...in fact, VERY pressurized, based on the symptoms he's described--to the point where pumping the toilet any more could rupture the tank if it doesn't cause the whole tankful to erupt back through the toilet first. Check first at the vent thru-hull...scrape out the fitting with screwdriver. If there's a screen in it, don't worry about destorying it...you're better off without a screen. Have a hose handy too, 'cuz if that's where the blockage is, waste will erupt out the vent. If that doesn't open it up, verrrrry carefully loosen the cap on the deck pumpout fitting--and hang on tight to it, 'cuz the pressure from a highly pressurized tank can spin it out of your hand and overboard. Have a hose handy here too, 'cuz the pressure will cause the tank to erupt...you'll see the first signs as soon as the cap is loosened at all. NOW you can check for a blockage in the tank vent connection at the tank...and if the thru-hull fitting wasn't clogged, the tank connection is almost certain to be...'cuz the two most common places are the thru-hull and the tank. Do NOT use the toilet again or even think of trying to pumpout or dump the tank at sea until the tank vent is clear. A strong pumpout pulling a vacuum against blocked vent can crack the tank...the macerator isn't strong enough to do that, but can't empty the tank when pulling against a vacuum either. In the future, cease all use of the toilet immediately upon feeling any backpressure. Check your vent thru-hull regularly...backflush the vent every time you pump out and/or wash the boat to keep that line clear. Do NOT use the vent as an "overflow" to let you know when the tank is full! Install a gauge if you don't have one, and cease all use of the toilet when the light indicates it's full. The link below is worth checking out too.