The tank has to be replaced..
Which means you're gonna have to do whatever it takes to gain access to it. Underneath the cabin sole is the worst possible location for a holding tank on a sailboat, so once you get it out let's try to find a better place for it. You can use that space for a water tank or storage if you want to make a hatch to go over it instead of closing up the sole again. To find a source for tanks, along with some guidance (although I'm not sure I'd ccall it "spiritual guidance") for this project, go to the Head Mistress forum and click on the link to the Reference Library (that's LIBRARY, not archive). Mid-summer is NOT the most pleasant time to have to do this job...but it can made a LITTLE less smelly....To get rid of the odor from the spill: after you get the tank out, wash the area with detergent and water...do NOT use bleach, Lysol, or any "germ killing/antibacterial" products. After it's reasonably dry, LIBERALLY saturate every surface with Raritan K.O., which is a live-bacteria holding tank treatment...not only does it work in holding tanks, but the bacteria "eat" odor causing spores. Don't rinse...just let it dry, leaving everything open so that plenty of fresh air can circulate, for a full day. Give me a shout after you've gotten the tank out and I'll help you spec out a better system.