I intended to bring this up the other day, but forgot until last night when I noticed someone again mentioning the lack of bathrooms at Collinwood.
I knew there was at least one time, perhaps a couple when there WAS an indication of their existence. The other day when David returned to his room to find Maggie waiting for him, his initial explanation was that he had gone to get washed. I forget if he mentioned face and hands or just hands. But, it's a clear indication that they really do have at least one bathroom in the place. Clearly it's a topic that won't normally come up, but if there's a separate room for him to go and wash his hands, I think it quite reasonable to assume it's a full bathroom!