Clogged Black Tank

You gotta always have some water in there. That stuff will dry and turn to concrete or maybe it's crapcrete. Anyway, water is your friend.
I have added lots of water just to dump. I knew not much was in there but wanted to dump before getting home. I like to have a strong swoosh of a dump so add water, then dumped, add water again for another dump then add water again to go home.

That is bad juju. In over 13 years of RVing I never had this problem as I paid extra attention to this possibility. Just lucky I guess.

OP must have fell in :B

Don't eat so much cheese.

Well, it appears he at least got it drained & forgot to come back.

Or he's still sputtering, spitting, and cleaning up.

We need to know.

Two stories both true First the one I hand a hand in.
Toilet would not drain indicating tank full. only it was not it was empty
Toilet down pipe was clogged.

And now the other one Same symptom as you
Well he finally got the tank empty (He did not say how) and investigated.

Turns out the builder when they cut the hole in the floor to install the toilet down pipe rather than toss the half moon cuts in the nearby trash. Just dropped them into the tank... They were returned to the company CEO.

Now.. if you are sure it's not the toilet.... A "Porta Potty" service can suck out the tank. Then it might be possible to inspect with an endoscope (you can get 'em that plug into your cell phone)

I assume that you dont have a built in black tank sprayer. buy one,let tank slowly empty below area near tank top l drill a hole in the wall of the tank opposite the drain end that is slightly larger than the tank sprayer assembly. beg/borrow/or steal a pressure washer, jam it into the hole in the wall and blast away. then mount sprayer assembly.
bumpy

Nothing beats a hard drive with lots of turns IMO.

Don't I remember a post from you about replacing your valve in early August? The problem may stem from that work.

As others have noted, make sure your valve is opening all the way. If it is, there are a couple of other possibilities. First (most likely) you have a big wad of TP sitting at the exit pipe in your black tank. Solution for this is to keep filling your black tank with water until it pushes the wad out. Second, you might have a tank cutout that floated to the exit pipe. For that you'll need to remove the valve, then remove the cutout with a pair of long nose pliers (messy job). Third, if you really do have a poop pyramid you could try a chemical like Tank Techs RX or a drain bladder like this (pushed down through the toilet): Drain Bladder

"IF" you have verified the valve is open and you think there's something stuck you can buy a cap for your outlet connection that also has a hose connection like this one.Sewer cap
You can then connect a garden hose and turn the water on just a bit and see if the water flows back into the tank. If it does you can turn it on a bit more and hopefully get whatever is there out. You will want 2 people so you can push the valve closed at the same time someone is shutting off the water and there will only be minimal mess.

From inside, if you open the toilet flush valve, can you see anything? If you pour a 5 gallon bucket of water down the toilet do you get 5 gallons out pretty quickly or is that where you only get a trickle?