I'd say your tank vent is definitely blocked
And that you're VERY lucky that the pressure in your tank didn't burst it. If you've used the boat for the entire season and have never pumped the tank out, it's no wonder that the vent is blocked...'cuz the tank has been overflowing out the vent most of the season--it's been full since prob'ly the 3rd or 4th weekend aboard...Sooner or later the buildup of waste in it had to clog it. The most likely places are the vent thru-hull and the vent hose connection on the tank. I doubt that the pumpout was actually able to empty your tank against blocked vent. You lost enough to relieve the pressure when you opened the deck pump fitting...that also allowed some air to replace it...so at best, the pumpout could only remove as much as it could until the air in the tank was exhausted--a gallon or two at most, at which point it pulled a vacuum that prevented it from getting any more out of the tank. How much is still left in your tank depends on how much spilled when you opened the deck fitting. Do NOT use the toilet or attempt to pump out the tank again until you've cleared the vent!!! A strong pumpout pulling against a blocked vent is even more likely to crack a tank than than pressurizing it. And once you've cleared it, check it regularly and backflush it every time you wash the boat to mak sure it never becomes clogged again. There are two ways to know when the tank is full: check it visually, or install a gauge. Since you haven't thought it necessary to pump out all season, it's obvious you don't have a clue about how long it takes to fill a tank...here's the math you need to do it: Assuming that you have a manual toilet (which prob'ly needs some maintenance too, but that's another subject), the average flush uses about a 1/2 gallon (64 oz) of flush water (that can be cut way down if you know how, but that too is another subject)...the average waste "deposit" is about another 8 oz...the average adult uses the toilet 5x day. So each person aboard will put an average of 72 oz x 5 uses/day into the tank, or 360 oz. A gallon, btw, is 128 oz., so that means each person will put an average of 2.81 gal/day into the tank. Assuming a 20 gallon tank, and two people aboard, that means the tank MIGHT last two weekends--and that's only if you use the marina facilities a few times. I dunno know what size your tank is, but using the math above should tell you about how long (my guess: since sometime during the Memorial Day weekend) you've been overflowing your tank out the vent. I'm only surprised it took this long for the vent to clog. The link below will teach you what you need to knw to prevent most "head aches" in the future.
