The Lance 1055 I'm picking up soon has a door in the same place for the generator (no generator though) and when I went to look at it the door itself had water in it and there was a small puddle in the compartment. Buzzcut has the same camper and had the same issue so he sealed around the compartment door frame and the door itself. Problem solve and I'll be doing that to mine too. From the photos is looks like the worst rot is up against the frame of the door, so I would suspect the same issue and after the repair would take out the door/frame and check/seal it. Best of luck on your repair!
Dak