The dreaded black triangle of whatever it is....

You may not need enzymes. Plain old dish hand soap does a very good job at removing and dissolving grease, and poop is quite similar. Might just squirt some of that, add water, let sit, and it might just do the job.

So far, the best way I've seen of cleaning a black tank was an access panel on top, then going at the inside with a wet/dry vac, mop, and a plastic scraper.