In the episodes where Sam came to live with them Kimberly was off in Paris so did Willis use Kimberly room since sam moved in with Arnold? My guess is he did but who knows because it was never said or mentioned what room he used after Sam moved in.

I dont think willis would feel like a man if he was sleeping in a girls room it would be an awkward experience for him, i am sure his friends would pick on him if they found out, i think drummond had a guest room willis could sleep in.

http://www.youtube.com/watch?v=0KflQ5--uSY
http://www.youtube.com/watch?v=RGJRkTIZAmQ
http://www.youtube.com/watch?v=1cVcLoAjqHw