You definitely have a sewer stoppage, but is the problem between the house and the septic tank, or beyond the septic tank in the (possibly) over-saturated leach field?
Professionals will tell you to have the septic tank pumped every 5 years. The Rid-X is good to use, but you're beyond that doing you any good now.
If the tank has not been pumped in over 5 years, I would call around and get estimates to have it pumped. When they pump it, they have to dig it up (to access) and remove the lid, which is heavy concrete and covers about 1/4 of the tank on the inlet side. When they remove this lid and pump the tank out, they will inspect the inlet baffle tee (solids can collect here)- if the tee is clogged (hopefully) then you have your stoppage cleared and your tank pumped in one shot, however...
...the problem could be more serious, there could be roots (or a crushed pipe) causing the stoppage between the house and the septic tank, then this problem will have to be remedied in addition to the tank pumping.
The worst case scenario would be a saturated leach field, which is where the gray water goes to soak into the ground, then evaporate. If the field is oversaturated or the (gravel) in the leach field is clogged with dirt, it could be a costly situation