A simple initial step must be taken to distinguish between an in-building plumbing problem and an (outside) on-site waste disposal system problem.
Simply put, if a single building fixture is sluggish or clogged, but if other building fixtures drain properly, you should suspect a local clog or vent problem at the individual fixture.
If all building drains are slow or clogged, or if waste is backing up into the building from the lowest plumbing fixture, you would suspect the onsite waste disposal system.