[footfootfoot]

It sounds like your septic system is well beyond the help of bacteria/yeast solutions. My septic guy says that stuff is essentially like the snake oil you add to a fucked up engine; it's not a valve job.

If you can get a look at your waste pipes, check to see that any 90 degree bends are long sweep and or sanitary Ts. I know you are renting, but if you see poor plumbing that will explain a lot of your problem if it isn't just a septic tank failure. The tank should be filling with solids and the liquid should run off into the leach field. If the tank is over flowing then the liquids are not making it through the leach field or the ground isn't taking up the water. The leach field should be below the frost line and not backfilled with clay. Assuming the leach field is not the problem, another BIG culprit in plumbing problems is "flushable" wipes. They are not. They shouldn't be flushed. My BIL has made a lot of money cleaning out drains that were clogged with flushable wipes. They can be what's blocking the inflow into the tank.

The point about checking the quality of the visible waste plumbing is that it may be an indication of the over-all quality of the work.

Sorry you are having this particular adventure.