I take it that you parked a functional Airstream somewhere, and hooked a sewer hose to it, and left the black tank gate valve open?
If so, then quoting thenewkid64
, you probably have a "tower of terror" in your black tank.
Your RV toilet does not drain through a pipe like your home toilet does. When flushed, your RV toilet dumps into a tank where the solids more or less stay put, and the liquids drain off through the open waste gate. In time, quite a tower of... solids can form. Given enough time, the tower can block drainage.
The best thing at this point would be to shut the valve & fill the black tank up with water. Let it sit a few hours & then open the dump valve. Repeat until, with the aid of a flashlight, you see nothing in the tank while holding the toilet's foot pedal down.