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.

I've thought about bleach because it's more corrosive, but I'm worried what that would do to the any rubber gaskets and seals. Time to let it soak with an enzyme and get out the pressure washer.

Yes it'll take time.
I took my garden hose with a high pressure tip, the kind that screws on with no controls,it ran through the window of the bathroom, the garden hose has about 55 to 60 psi pressure, I took the toilet cleaning thing and propped the valve open enough to allow the hose in about a foot (My pipe is straight down)then I turned on the hose for about 2 or 3 min. and dumped the tank, the idea being high pressure works good--just don't go away and have a beer!!!!

Glad I have full hookups at home. This may take a while......

".... the black tank developed this rock hard and large deposit...."

The dreaded POOP PYRAMID.

Gonna need to bust it up......awning rod, pole etc.
Then you need to fill tank full and dump
Then fill it about 1/2 full and dump in a bunch of digester and let sit for couple days Digester then fill and dump again.

Then add 5 gallons of water and dump 10-20# of ice cubes into tank via toilet and immediately go for a drive over rough/curvy roads.
Fill and dump again.

Repeat until POOP PYRAMID is gone.

After you get rid of it...use more water!
Fill toilet bowl full when flushing poop
Don't dump tank until almost full (leave drain valve CLOSED until time to dump)
Flush/rinse after dumping
Add couple gallons back into tank so bottom stays wet and stuff doesn't stick