You could make a double sided material that is Transparent on the back-face and opaque on the front-face. This will allow light to pass through without being to see out. I've done something similar (actually the reverse) for the ceilings of units so that I could get a sense of lighting by windows by look at the unit from above.

check your memory usage. SketchUp is a 32bit process, so if you get close to 4gb of ram being used, you're going to get a crash. One day SketchUp will join the 64bit world, and this won't be a problem. Until then, try to keep RT renders in lower resolutions if you can.