So I hope it's clear my goal is to do something as quickly as possible without being completely wrong (if my objects turn out to have 10 times more vertices than a proper way, I might have to slow down a bit & take the time to create a bit better).
My first task is to create and empty structure where I'll put a camera ; basically a room where in my final project the starting character head will point as the camera. So I've created a cube then one a bit smaller then another quite smaller ; the 1st is the object Exterior, 2nd Interior, 3rd Door. Then I duplicated the Exterior and names it room, with which I'd apply Boolean Modifiers, then I disabled the original (Exterior). In this image, I selected interior to show that Door goes all the way inside Interior.

I then made the difference of Room with Door then hid the later. This image show the result.

I then wanted to scoop out the interior from Room (as in last step) by make a difference. The result isn't at all what I'm expecting, it's just the door frame.

I was hoping to have a room by now. Just like if the last step I took a circular blade (used to install a door handle) into a large watermelon and this step I'd just scoop the yummy inside ; ending with an empty shell with a hole.
The files are available on my homepage: http://dynv.qc.to/content/blender_Empty_object
Thank you kindly for you help