Toilet won't flush

Yesterday when I flushed it I could see that the water level was close to the bottom of the valve, so I drained the tank.

You tried to drain the tank, right? If you look down the toilet, it's still full of water, right? So you did not actually drain your tank. Your black tank is full.

Also, you should know that the gray water flows into the black water pipe above the main slide valve. So I can tell that the main slide valve is still working by draining the gray tank, and it is passing fluid.

It would be helpful to see a picture of this situation. On my RV, the black and grey water waste valves are arranged on either side of the exit pipe.

The way it works is you pull the black water valve while leaving the grey water valve closed, so all the human waste runs out, then you close the black water valve and open the grey water valve and this way your poop slinky pipe gets flushed out by the grey water.

This is what mine looks like:

For sure, I am not taking the hose of the pipe and running the snake up it. If the tank is full and it appears to be that would make a mess I never want to see, and I'm sure my park manager feels the same.

OK, but even with your poop slinky pipe connected, if you open the black water valve on a full black water tank you should have no doubt that liquid is GUSHING through that pipe. Moreover, on my poop slinky pipe the end elbow that hooks to the sewer is translucent and I can see liquid going through it.

You have to figure out whether anything is coming out when you open the black water valve. Do you have a cable-operated valve or is it a direct push rod like the picture above?

Steve