There isn't a direct analogy in Blender for Radiant style 'unfocused' movements so you have to more-or-less do what Ratty suggested and either clip the scene or show/hide elements from view ("H" to hide, "Alt+H" to unhide) as you work so you can see the interiors of structures, it's not ideal but I doubt anything is going to change in this regard (AFAIK no other 3D app do this, except level editors). Using "Ctrl+F" is the equivalent of 'free' mode in Radiants camera view (when you LMB+hold, drag the mouse around, it's equally as sensitive as well). What you might also want to do is snap the camera to different objects in a room, that will then mean you can change the point of focus to different areas/objects as you work.
