You have a remedy. It is in the RLTA. RCW 59 18 060, RCW 59 18 085 and RCW 59 18 115. You start with written notice to your landlord of the issue with the sewer. You specify exactly what is wrong, and you ask them to fix it. They have 24 hours to respond. If they don't respond, contact the building department, give them a copy of the letter you wrote to your landlord, and demand an inspection within five days. If the building department agrees with you, and advises your landlord to fix the sewer, you will be able to get your landlord to pay you to move.
I have written a legal guide on this subject, and I hope you read it and I hope it helps. However, haring off to the Health Department or the EPA will not do anything for you. You don't need the landlord to cooperate, you need to know what the law requires. Hope this helps. Elizabeth Powell