Welcome to Just Answer.
Sorry to hear about your problem.
Sewage backing up into the tub/ shower is a sure sign that you have a partial clog in the main drain pipe and the waste drain water is taking the path of least resistance and backing up into the tub or shower drain.
To clear this blockage you can use a plumbers snake/ auger in the main drain pipe or you could purchase the Drain King. It can be found at your local home improvement store or hardware store; they work great at unclogging drains. It comes in different sizes for different drain diameters and you simply attach it to a hose and slide the hose down the drain pipe as far as it will go. You then turn on the hose and it builds up pressure to the point where it releases in several bursts, expanding and contracting. You can clear one obstruction and then push the hose further to get to any others and they are only around 10 to 15 dollars.
I would also verify that the vent piping is clear. Any air bubbles/ gurgling are a sure sign that the main vent pipe can be partially clogged from an animal getting caught or a bird building a nest, snow/ ice, etc. This would effect proper draining as well. The vent sucks in air and replaces the displaced water, allowing it to drain properly; it can be compared to holding your thumb on the end of a drinking straw that is full of water; the water will stay in the straw, until you lift your thumb, allowing the air in. A high pressure nozzle, on the end of a water hose, can be used up on the roof to spray down the main vent pipe. This is effective at clearing blockages in the vent pipe only; often the vent pipe can be augered as well. See short video below for more information.
And lastly, if your septic tank has not been pumped in the last 3-5 years, then it is time to have it pumped. This is a regular preventative maintenance item that should be done every 3-5 years to keep your septic system working efficiently.
If none of these work or you simply do not feel like doing the work, then you can have a plumbing company come out and clear the clog for you, some have a camera that can be sent down the pipe for serious clogs. There could be tree roots taking over the pipe, a disconnection in the pipe, or the pipe could be sloping in the wrong direction. All of these would take some additional work to fix, but at least you will know the problem and can go from there.
If you need further help or clarification on this answer, please do not hesitate to reply to this post.
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Best of Luck, Brian
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