If it's so full it's "ready to explode," it should be overflowing out the vent...unless the vent is blocked. And if waste is backing up into the toilet, if you can't flush, it's pretty good indication that it is blocked. You do NOT need to inspect the tank to find out....if flushes aren't going down, it definitely is.
All tank vents have two main functions: to provide an escape for air displaced by incoming contents and provide a source of air to replace contents as they're pulled out. When air displaced by incoming waste cannot escape out the vent the tank becomes pressurized, creating increasing back pressure that prevents the toilet from flushing.
A blocked tank vent won't let you pump out either..'cuz without a source of air to replace contents as they're being sucked out of the tank, the pump will pull a vacuum that'll prevent it from pulling out more than a gallon or two. A particularly strong pumpout can even implode a tank.
So do NOT use the toilet again or try to pump out or dump the tank until you've cleared the vent. This can be done without gaining access to the tank.
The two most common locations for a vent blockage are the vent thru-hull and the other end of the vent line--that end of the hose and the vent fitting on the tank. Start by cleaning out the vent thru-hull--or if your tank vents out holes or slits in a rail stanchion (common on Catalinas), clean out the holes/slits in the stanchion. Use a screwdriver blade, ice pick--whatever works. If that doesn't result in a spew out the vent, you'll need to gain access to the tank to clean out the vent fitting on the tank and that end of the vent line. But you'll have to relieve the pressure before removing the vent line from the tank to clean them out...which means you'll have to open the deck pumpout fitting VERY CAREFULLY with a hose at the ready...'cuz there WILL be a spew and I can't think of anything you can do to prevent it. So you might want to do this away from the dock and any other boats...and make sure the pumpout fitting is the on the lee side of the boat and you're UPwind of the fitting!
Once you've relieved the pressure, you'll be able to flush what's in the bowl. But don't use the head again yet. Other C-32 owners here should be able to tell you how to get that panel off. And I'll be glad to help you learn the rest of what you need to know to prevent toilet and tank problems in the future.
--Peggie
