Sorry, Uwe. This won't help you, but others reading this thread might be interested to know that the PO of my Argosy (or Airstream factory?) had installed screws up against the toilet bolt heads to keep them from turning when the nuts were removed.
Another thing the PO did was not so nice. He apparently had a leak around the toilet flange. To fix this he glued the flange to the black tank. I broke my black tank trying to remove the flange.
I'd be rather upset except my black tank had serious cracks causing it to leak anyway. That's why I was removing it.
Now, if someone out there knows how to fix the cracks in a leaky plastic black tank... I'm not listening... hmmm....hmmmm...hmmmm