I had a maytag washing mashine before, and I replaced by a Roper vertical.
I can see a back flush I guess, when the washing mashin is in use. The washing machine is in the far right side of the house and we can not use the toilet on that side of the house.
Do you think the septic tank is full or I have to do something different

I see you are new here. Welcome to this site. Sounds like you have a plumbing issue. If you would also post in plumbing forum, you may get more response. Some good folks there that really know there stuff. Good Luck