Recently I've been having the back of my mouth so dry. I do leave the fan on at night, can that be the cause or am I not drinking enough water?

Dry mouth. If the dry mouth is occurring at night, one of the common causes is allergy with nasal obstruction/congestion, which forcs you to breathe through your mouth while asleep (you are not aware of this), which leads to very, very dry mouth in the morning. Sleeping in a non-humidified room can also do it.