Check the cleanout going to your septic tank. If the water level is high there then you may have a clogged pipe at the tank or it may be time to get it pumped (I had a root ball grow into mine a few years ago). Another thing to check is your vent stacks. There should be one no more than a few feet from each toilet. Running a snake through those can help sometimes. Good luck.
@David: Make the owner fix it.
