Does the tank have an inspection port?
When tanks are overfilled, especially if the vent is blocked, they will leak out the inspection port. That you were only able to pump out "most of the tank" is a pretty good indication that your vent IS blocked. Don't try to pump out your tank again until you're sure the vent is open--an onboard macerator isn't strong enough to crack a tank, but a pumpout can....check at the thru-hull first...that's where 99% of vent blockages occur. Clean up the mess with plenty of detergent and water. Do NOT use bleach...it won't get rid of the odor permanently. Stop by a WM or B/US and pick up a bottle of Raritan K.O. holding tank treatmnet and a trigger sprayer. K.O. is a live bacteria product...the bacteria in it "eat" the odor-causing spores. After you've gotten the mess cleaned up and the area thoroughly rinsed and reasonably dry again...LIBERALLY spray K.O. everywhere, making sure to get under and behind anything. Don't rinse, just let it dry...leaving all the hatches and lockers open so that plenty of fresh air can circulate for 24 hours. Even put a fan in the boat to increase fresh air circulation. If that doesn't get rid of all the odor, you haven't found all the spill yet. To prevent future problems, read the articles in the Head Mistress forum reference library. And as soon as I find the time to dust it off, the following (which was a reply to someone on another site) will be added to library:A tank cannot BE emptied under vacuum. Unless air can be pulled in through the vent to replace the waste (or anything else in ANY container) being pulled out, a vacuum is created in which nothing more can beremoved--but in which the suction CAN crack the tank. A blocked vent also prevents the air displaced by incoming contents from escaping, which results in a pressurized tank. This can have disastrous consequences--anything from blown out fittings to a cracked tank to a sewage geyser when the pumpout fitting cap is removed to a backup in the toilet. Follow these "rules" to prevent the above problems: 1. check your vent through-hull regularly...blast it out with a hose every time you wash the boat;2. Never turn pumpout operation over to a dockhand and walk away. Stay there and watch to make certain that the tank is being completely pumped out. Terminate pumpout/dumping at sea immediately if waste stops flowing before the tank is empty. Do not try to pump out or dump the tank again until the vent has been cleared.3 Stop flushing and cease using the toilet when you feel any backpressure or see any waste backing up into the toilet.4. The vent is NOT an overflow. Only air should ever pass through it. Accidents do happen, though, and they can result in a clogged vent. Cease using the toilet immediately if you see waste coming out the vent and and get to a pumpout as soon as possible so that waste doesn't have time to dry and harden in the vent hose. Flush it out VERY thoroughly with clean water to remove any bits of waste in it. To prevent overflows out the vent, install a gauge and pay attention to it.