Alright so I did the same thing last night with my RT left it at 32% at 11:00PM woke up at 6:00AM and the battery was 8%. A little more than 2% this time. All apps were closed in the background. Bluetooth, Wi-Fi both off. Screen brightness less than half. I don't know why it lost so much this time. I did leave it the kickstand on with the touch cover but would that make a difference? Should I leave the touch cover closed when I'm not using it?
Are you on RT (RTM) or RT 8.1 Preview? If you are running the preview turning WiFi and Bluetooth off actually drains the battery significantly more due to the system being unable to invoke Connected Standby, so basically it ran all night.