What you are basically asking is how to "gaslight" someone (wikipedia article on the topic: https://en.wikipedia.org/wiki/Gaslighting) and the way I would do it is make the roommate question their own memory. Therefore, leaving reminder notes of things the roommate would easily remember should work. Ideally you should do an excellent job at forging your roommate's handwriting for the reminder notes. Failing that either typewritten notes or use a writing style neither you or your roommate use so you can also add the possibility of split personality along with forgetfulness to the gaslighting scheme.
Example reminder notes would be to eat meals, reminder notes for what the roommate's favorite TV show is and also an occasional commonsense reminder note such as water is wet.
Leave the notes scattered around but never mention them unless your roommate asks about them. Just act like having those notes scattered around is an everyday thing that you don't pay attention to because it's perfectly normal. When your roommate asks about the notes then just say something like, "How should I know? They're your notes.".