This happend to me a few years ago. Toilet was full but nothing came from black tank when slide was opened. With us, was a Camco RV Flexible Swivel Stik with Shutoff Valve
https://www.amazon.com/s?k=black+tan...-mkt-opr-us-20
...that I was able to put into the schmuck in the toilet and pushed toward the drain end of the tank. That pushing was enough to break the "damn" and empty the tank. Once we returned home we used this product to liquify the remaing Kling-ons:
https://www.amazon.com/Unique-RV-Sen...s%2C123&sr=8-9
After letting it sit in the tank for a few days, we headed to another camping trip and dumped the contents of the tank at a rest area with an RV Dump Station on the way.
Hope your results are similar!
Murff