Here's a new config I've created using the new editor.
The service catering doors both work fine.
The rear cargo door I had to choose the small AKH (LD3-45) ULD, because the door doesn't open very high. I tried to find out what is loaded into this door but couldn't find any photos. You might prefer a belt for this door.
The front cargo door correctly loads the AKE (LD3) that the A300 is supposed to carry.
For the passenger doors - I have set the main exit low so that stairs will not come - use the simcheck stairs.
I left this door turned on so that you get reminded to open the door if you forget when boarding.