Black Tank only holds 1/2 to 1/3 of capacity

Sounds like you have a large version of the poop pyramid going on. Someone left the black tank draining during use. Some form of pooper-mache.
Choose a good bacterial poop/paper cleaner and put that in the tank, fill it with water, let it set a few days, take it for sloshing drive to really stir it up and let it set a few days more. THEN dump it and wand it out.
So far my fav cleaner is Happy Camper.
Many swear by the ice cube trick. Best bet there is to make your own large solid cubes so they have chance to knock things around and apart before melting. But if the contents isn’t softened a bit before, it won’t work that well. Also when using ice cubes you don’t want a tank full of liquid. You want the cubes knocking around the bottom.
IF you have a direct drop into your black tank, you could lightly use a pressure washer to break the pyramid apart. The reason I say lightly is BACKSPRAY and high pressure water could eat into the tank plastic.

TexanSon wrote:
… (as well as shining a light into the toilet with the valve open, and seeing liquid just below the floor). As for the flexible hose wand, that is what I use.

If you really have water to just below the floor with the valve open, then suit up; because you are going to get really wet, dirty and create a big mess.

I know the sensors are only suggestions, when they work, but when I can get the sensor light to show 1/3 full, put two 5 gallon buckets of water in, and it shows that it is full, I'm pretty sure that's an indication (as well as shining a light into the toilet with the valve open, and seeing liquid just below the floor). As for the flexible hose wand, that is what I use.

Use a flexible hose wand and flush for an extended time.

The primary purpose of the idiot light gauges is for the salesman to explain about the great monitor system. Why do you think it's full with 10 gallons?

Fill with water and drive. Add bags of ice cubes and drive.