It doesn't really matter what address you list as long as it somehow identifies the rental unit. (E.g. 123 main street, unit B or 123-B main st, top floor.). No automated process is looking at the addresses, certainly at that level. You just want it to be clear should a human being ever look at the return and have to compare some supporting documentation (e.g. to help indicate that a receipt for a repair to the rental unit was indeed for the rental unit and not the non-rental unit). If the US Mail is sent to a particular address for the rental, I might suggest using that.