Plumbing Questions? Ask a Plumbing Expert.
Tell me why you think its not a fully loaded and not draining properly cess pool (septic tank)... was that inspected or pumped... was the drain field tested?
Its either a non functioning septic tank and drain field, or tree roots or other clog in the sewer main blocking the flow.
the fact shredded toilet paper water under the house points to a missing or broken vent pipe, or broken sewer line.
Let me know about the septic tank and drain field cleaning if any has been done and what the results were.
Some photo's of the sewer line under the house would be helpful, use the paper clip icon at the right end of the tool bar to attach those.
We can go from there.