My lessons learned are: 1) I need a damn gauge on that tank; 2) I need to pump out every other weekend regardless of use; 3) I need to assume the pump out boat has not been by, unless I am there to actually see it.
Not the right lessons! The most important one: Maintain your tank vent! A blocked vent is source of all your problems.
I'd bet the pumpout boat HAD been there, but couldn't pull anything out of the tank because the vent was blocked. 'Cuz unless there's a source of air--the vent-- to replace tank contents as they're drawn out, the pump will pull a vacuum that prevents anything from being pulled out. A larger vent wouldn't have changed that, although a larger vent line and an open thru-hull instead of a "vent" thru-hull can make it easier to maintain the vent and help to eliminate odor out the vent. You're lucky that the pumpout boat's pump wasn't strong enough to implode the tank or that continuing to use the toilet didn't pressurize the tank enough to burst it.
I'm also betting that you and other members of your family were aware level that the toilet was getting harder to pump if a manual toilet, or"burping" and spitting up if electric as the system became increasingly pressurized, and may even have told you it was...but you ignored it. So every use of the toilet increasingly pressurized the system...to the extent that it had to take a LOT of effort to pump it or y'all were seeing a serious amount of reflux.
So the second less you need to learn is two-fold: 1. A toilet that's getting harder to pump (or "burping" and spitting up if it's electric) is the first sign of a blocked vent tank vent.
2. Everybody needs to start paying more attention to even the slightest changes in the way equipment--ALL the equipment--on the boat behaves...so you can catch small problems that are easy to fix before they turn into big expensive problems.
A gauge on the tank (the Profile gauges are the best
Profile Tank Monitors ) is a good idea, but wouldn't have prevented your eruption. Neither would pumping out every weekend because if the vent is blocked you wouldn't be able to pump anything out.
So...do not use the toilet again until you've cleared the vent line. The most common location for holding tank vent blockage is the thru-hull...Use a screwdriver blade, ice pick or whatever works to scrape it out...if there's screen on it, cut it out...screens cause more problems than they prevent or solve. The other most common location on a sailboat is the vent line connection to the tank...both the vent fitting on the tank and that end of the hose. Scrape 'em out. I suggest you o[pen the deck pumpout cap to relieve any pressure before you remove any hoses.
This could be a good time to upgrade to a 1" vent line and an open "bulkhead" thru-hull. Easy to do...all you need are a hole saw, some hose, a threaded plug (available from any hardware store) for the existing vent fitting on the tank, and a li'l doodad called the Uniseal
UNISEAL That'll give you a vent that'll be very easy to maintain...just stick the hose nozzle up against the thru-hull and back flush the line every time you wash the boat.
Oh...when you clean up the mess, you're likely to have residual odor, especially in hot weather. DO NOT USE BLEACH! Clean with detergent and water...rinse well...let the area get dry. Then spray every surface, nook and cranny that was affected with a product called PureAyre
PureAyre ...just a light coating is enough...and let it dry, leaving hatches and/or lockers open so that plenty of fresh air can circulate for at least 24 hours. The stuff WORKS. The online store right here sells it:
http://shop.sailboatowners.com/prod.php?11608/
If you need more help, send me an EMAIL and we'll work out a good time to talk instead of all this d'd typing!
