sans_fi said:
Curiously enough, 1981/82 was also the date when he got lazy, and decided to wrap the story with one movie (Return of the Jedi) instead of doing the whole second trilogy with Luke searching for his sister etc, and fighting the Empire for 3 more movies.
That's not really what happened. Read The Secret History of Star Wars. Appendix D, The Legend of the Sequel Trilogy, examines all the rumors and myths, as well as Gary Kurtz's claims that are often simply taken as the gospel truth about the sequel trilogy.
The sequel trilogy was not condensed into ROTJ. There doesn't seem to have been a set plot for it (remember, Lucas was making this stuff up as he went along). "No, there is another" was originally supposed to be a vague setup to this sequel trilogy, and didn't have anything to do with Luke having a sister.
Because "the other" turned out to be Luke's sister, some people conflate the mention of Luke's sister in the first draft of ESB with "No, there is another." I think Kurtz may have been conflating various ideas that existed at various points in time of what was to happen in the sequel trilogy - the possibility of Luke's sister showing up in later episodes (c. first draft), the possibility of "the other" showing up in later episodes (c. Kasdan's drafts), but I don't think there was ever a single set plan that was "totally tossed out the window" by Lucas, as Kurtz claimed in the Film Threat interview.
As for not going to Tunisia, I'm sure it was budgetary reasons, since Tatooine exteriors comprised a much smaller part of the film, and it wasn't seen as necessary to pack up the whole crew to another far-flung location shoot for only a few scenes. (And I'm not sure that Lucas, Howard Kazanjian, or Harrison Ford would have relished another trip to Tunisia so soon after the torturous production of Raiders...)
Lest we forget, Empire was an amazing film, but it was a grueling production, the shoot ran overtime and went overbudget, and Lucas went through much stress and anxiety. Again, read The Secret History of Star Wars.